java并发编程（八）synchronized原理之偏向锁

作者: 我犟不过你 | 来源:发表于2021-11-29 17:15 被阅读0次

偏向锁的引入，对轻量级锁的CAS操作进行了优化。轻量级锁需要每次都是用CAS来尝试获取锁，占用CPU时间。

jdk1.6中引入偏向锁后，只有第一次使用 CAS 将线程 ID 设置到对象的 Mark Word 头，之后发现这个线程 ID 是自己的就表示没有竞争，不用重新 CAS。以后只要不发生竞争，这个对象就归该线程所有。

一、偏向锁的状态

|----------------------------------------------------------------------------------------------|
|                                   Mark Word(64bits)                     |      State         |
|----------------------------------------------------------------------------------------------|
|    unused:25|identity_hashcode:31|unused:1|age:4|biase_lock:0| lock:01  |      Nomal         |
|----------------------------------------------------------------------------------------------|
|    thread:54|      epoch:2       |unused:1|age:4|biase_lock:1| lock:01  |      Biased        |
|----------------------------------------------------------------------------------------------|
|                        ptr_to_lock_record:62                 | lock:00  | Lightweight Locked |
|----------------------------------------------------------------------------------------------|
|                       ptr_to_heavyweight_monitor:62          | lock:10  | Heavyweight Locked |
|----------------------------------------------------------------------------------------------|
|                                                              | lock:11  |    Marked for GC   |
|----------------------------------------------------------------------------------------------|

如上表所示，偏向锁的状态biase_lock是1，lock是01。

下面我们借用jol工具看下状态：

<!-- https://mvnrepository.com/artifact/org.openjdk.jol/jol-core -->
<dependency>
    <groupId>org.openjdk.jol</groupId>
    <artifactId>jol-core</artifactId>
    <version>0.16</version>
</dependency>

1.1 默认情况下的偏向锁

引入依赖后，有如下代码：

    public static void main(String[] args)  {

        Object lock = new Object();

        System.out.println(ClassLayout.parseInstance(lock).toPrintable());
    }

结果：

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000000000001 (non-biasable; age: 0)
  8   8        (object header: class)    0x000000001be01c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

我们只关注上述结果的mark word 部分，后面的括号当中的内容是non-biasable，就是没开启偏向锁。并且value的值是001结尾，对照上面的Mark Word 状态表的后三位，刚好是无锁状态，0,01。

此处有一个问题，偏向锁是延时的，在刚启动的时候并没有开启，我们可以使用以下参数设置立即生效：

 -XX:BiasedLockingStartupDelay=0

设置完参数后再次启动的结果如下，发现16进制的value是5，转为2进制就是101，是上表中偏向锁状态的后三位，并且括号内的内容变成了biasable，具有了偏向锁的能力：

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000000000005 (biasable; age: 0)
  8   8        (object header: class)    0x000000001c0a1c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

总结：
- 偏向锁是默认开启的，但是是延时的，需要通过参数-XX:BiasedLockingStartupDelay=0禁用延时。
- 对象在创建后，如果开启了偏向锁，thread、epoch、age 都为 0。
- 对象在创建后，如果没有开启偏向锁，hashcode、age 都为 0。
- 禁用偏向锁，使用-XX:-UseBiasedLocking参数。

1.2 线程持有偏向锁

1.2.1 单线程持有

当有线程持有上述偏向锁，那么对象头的Mark Word 会有什么变化呢? 代码如下：

    public static void main(String[] args) {

        Object lock = new Object();

        System.out.println(ClassLayout.parseInstance(lock).toPrintable());

        synchronized (lock) {
            System.out.println(ClassLayout.parseInstance(lock).toPrintable());
        }

        System.out.println(ClassLayout.parseInstance(lock).toPrintable());
    }

在锁被持有前，持有中，持有后的状态分别如下：

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000000000005 (biasable; age: 0)
  8   8        (object header: class)    0x000000001c081c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x000000000320a005 (biased: 0x000000000000c828; epoch: 0; age: 0)
  8   8        (object header: class)    0x000000001c081c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x000000000320a005 (biased: 0x000000000000c828; epoch: 0; age: 0)
  8   8        (object header: class)    0x000000001c081c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

如上所示，当锁被线程持有后，mark word的内容发生了变化。其中value是8字节的全部内容，末尾是005，转化成2进制就是101，确实是使用的偏向锁。在括号内的biased表示偏向锁在哪个线程上，后面的16进制数，就是线程的id。

在锁被释放后，当前对象仍然是持有该偏向锁的。

1.2.2 两个线程竞争

有如下代码，使用main方法的主线程和一个Thread模仿一个锁竞争的情况，看看对象头是如何变化的。

    public static void main(String[] args) throws InterruptedException {

        Object lock = new Object();
        // 打印初始状态
        System.out.println(ClassLayout.parseInstance(lock).toPrintable());

        synchronized (lock) {
            // 打印偏向锁状态
            System.out.println(ClassLayout.parseInstance(lock).toPrintable());
            TimeUnit.SECONDS.sleep(2);
        }

        TimeUnit.SECONDS.sleep(1);
        new Thread(()->{
            synchronized (lock){
                // 打印轻量级锁
                System.out.println(ClassLayout.parseInstance(lock).toPrintable());
            }
        }).start();

        TimeUnit.SECONDS.sleep(1);
        // 打印无锁
        System.out.println(ClassLayout.parseInstance(lock).toPrintable());
    }

结果如下所示：

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000000000005 (biasable; age: 0)
  8   8        (object header: class)    0x000000001bbd1c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000002e1a005 (biased: 0x000000000000b868; epoch: 0; age: 0)
  8   8        (object header: class)    0x000000001bbd1c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000020b5efe0 (thin lock: 0x0000000020b5efe0)
  8   8        (object header: class)    0x000000001bbd1c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000000000001 (non-biasable; age: 0)
  8   8        (object header: class)    0x000000001bbd1c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

过程分析：

新创建对象，开启偏向锁
当主线程获取锁后，独自持有偏向锁，阻塞2s
一秒后，另一个线程来竞争此锁，此时因为主线程在阻塞，所以此线程升级成了轻量级锁，如上结果thin lock表示轻量级锁，后面的16进制数表示持有锁的线程id。
当主线程再次休息一秒，同步代码块执行完毕，此时释放锁，所以最终变成了无锁状态non-biasable。

1.2.3 多个线程竞争

使用5个线程竞争：

    public static void main(String[] args) throws InterruptedException {

        Object lock = new Object();
        // 打印初始状态
        System.out.println(ClassLayout.parseInstance(lock).toPrintable());

        synchronized (lock) {
            // 打印偏向锁状态
            System.out.println(ClassLayout.parseInstance(lock).toPrintable());
            TimeUnit.SECONDS.sleep(2);
        }

        TimeUnit.SECONDS.sleep(1);

        new Thread(() -> {
            synchronized (lock) {
                // 打印轻量级锁
                System.out.println(ClassLayout.parseInstance(lock).toPrintable());
            }
        }).start();

        TimeUnit.SECONDS.sleep(1);

        // 5个线程竞争
        for (int i = 0; i < 5; i++) {
            new Thread(() -> {
                synchronized (lock) {
                    // 打印轻量级锁
                    System.out.println(ClassLayout.parseInstance(lock).toPrintable());
                }
            }).start();
        }

        TimeUnit.SECONDS.sleep(1);
        // 打印无锁
        System.out.println(ClassLayout.parseInstance(lock).toPrintable());
    }

结果如下所示：

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000000000005 (biasable; age: 0)
  8   8        (object header: class)    0x000000001b941c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000002ada005 (biased: 0x000000000000ab68; epoch: 0; age: 0)
  8   8        (object header: class)    0x000000001b941c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x00000000208df7f0 (thin lock: 0x00000000208df7f0)
  8   8        (object header: class)    0x000000001b941c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x000000001c33a60a (fat lock: 0x000000001c33a60a)
  8   8        (object header: class)    0x000000001b941c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x000000001c33a60a (fat lock: 0x000000001c33a60a)
  8   8        (object header: class)    0x000000001b941c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x000000001c33a60a (fat lock: 0x000000001c33a60a)
  8   8        (object header: class)    0x000000001b941c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x000000001c33a60a (fat lock: 0x000000001c33a60a)
  8   8        (object header: class)    0x000000001b941c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x000000001c33a60a (fat lock: 0x000000001c33a60a)
  8   8        (object header: class)    0x000000001b941c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000000000001 (non-biasable; age: 0)
  8   8        (object header: class)    0x000000001b941c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

前面的过程，与上一小节的测试结果没有不同，不同处在于后面的循环创建的5个线程，此时打印的对象头内容是fat lock，表示重量级锁，其后面跟着的16进制数表示Monitor的地址，多个线程竞争时，未获取到锁的线程将会添加到Monitor的EntryList当中，等待唤醒。

二、锁撤销

2.1 hashcode

在创建对象的时候，hashcode是0，只有当调用该对象后，hashcode才被赋值，如下代码所示：

    public static void main(String[] args) throws InterruptedException {

        Object lock = new Object();
        // 打印初始状态
        System.out.println(lock);
        System.out.println(ClassLayout.parseInstance(lock).toPrintable());
    }

结果如下：

java.lang.Object@16f65612
java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x00000016f6561201 (hash: 0x16f65612; age: 0)
  8   8        (object header: class)    0x000000001c051c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

轻量级锁会在锁记录中记录 hashCode
重量级锁会在 Monitor 中记录 hashCode

调用对象的hashcode()方法，会使对象的偏向锁被撤销：

    public static void main(String[] args) throws InterruptedException {

        Object lock = new Object();
        // 打印初始状态
        System.out.println(ClassLayout.parseInstance(lock).toPrintable());


        synchronized (lock) {
            // 打印偏向锁状态
            System.out.println(ClassLayout.parseInstance(lock).toPrintable());
        }

        //调用hashcode
        lock.hashCode();
        System.out.println(ClassLayout.parseInstance(lock).toPrintable());

        //再次获取锁
        synchronized (lock) {
            // 打印偏向锁状态
            System.out.println(ClassLayout.parseInstance(lock).toPrintable());
        }
    }

结果：

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000000000005 (biasable; age: 0)
  8   8        (object header: class)    0x000000001be21c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x000000000305a005 (biased: 0x000000000000c168; epoch: 0; age: 0)
  8   8        (object header: class)    0x000000001be21c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000002ed94a8b01 (hash: 0x2ed94a8b; age: 0)
  8   8        (object header: class)    0x000000001be21c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000002faf730 (thin lock: 0x0000000002faf730)
  8   8        (object header: class)    0x000000001be21c00
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

如上所示：发现调用hashcode后，对象头打印出了hash值，并且将value的值转换成二进制后得到后三位的结果是001，001表示无锁的正常状态。

当再次尝试获取锁的时候，发现获取到的只能是轻量级锁（thin lock）了。

2.2 偏向锁锁撤销 - 升级轻量级锁或重量级锁

这里就是前面偏向锁状态当中，当有多个线程竞争锁的时候，无论是升级成轻量级锁，还是重量级锁，在最终将锁释放后，都会变成无锁状态，即偏向锁的状态被撤销了。

2.3 重偏向优化

如果对象虽然被多个线程访问，但没有竞争，这时偏向了线程 T1 的对象仍有机会重新偏向 T2，重偏向会重置对象的 Thread ID。

当撤销偏向锁阈值超过 14 次后（这个次数在我的电脑时14，有些人似乎20，暂时不知道怎么设定的），jvm 会这样觉得，我是不是偏向错了呢，于是会在给这些对象加锁时重新偏向至加锁线程。

如下所示，有两个线程，t1组装一个大小为30的集合，结合内创建30个对象。t2在t1处理数据时，通过wait等待，当t1通过notify唤醒t2时，t2开始占有锁（30个对象，每一个都有自己的锁）：

    public static void main(String[] args) {
        test3();
    }

    public static String print(String s) {
        String[] split = s.split("\r\n");
        return split[2].substring(41);
    }

    private static void test3() {
        Vector<Object> list = new Vector<>();
        Thread t1 = new Thread(() -> {
            for (int i = 0; i < 30; i++) {
                Object d = new Object();
                list.add(d);
                synchronized (d) {
                    System.out.println(i + "\t" + print(ClassLayout.parseInstance(d).toPrintable()));
                }
            }
            synchronized (list) {
                list.notify();
            }
        }, "t1");
        t1.start();

        Thread t2 = new Thread(() -> {
            synchronized (list) {
                try {
                    list.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println("===============> ");
            for (int i = 0; i < 30; i++) {
                Object d = list.get(i);
                System.out.println(i + "\t" + print(ClassLayout.parseInstance(d).toPrintable()));
                synchronized (d) {
                    System.out.println(i + "\t" + print(ClassLayout.parseInstance(d).toPrintable()));
                }
                System.out.println(i + "\t" + print(ClassLayout.parseInstance(d).toPrintable()));
            }
        }, "t2");
        t2.start();
    }

结果：

0   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
1   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
2   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
3   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
4   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
5   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
6   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
7   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
8   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
9   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
10  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
11  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
12  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
13  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
14  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
15  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
16  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
17  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
18  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
19  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
20  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
21  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
22  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
23  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
24  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
25  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
26  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
27  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
28  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
29  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
===============> 
0   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
0   0x00000000210df370 (thin lock: 0x00000000210df370)
0   0x0000000000000001 (non-biasable; age: 0)
1   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
1   0x00000000210df370 (thin lock: 0x00000000210df370)
1   0x0000000000000001 (non-biasable; age: 0)
2   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
2   0x00000000210df370 (thin lock: 0x00000000210df370)
2   0x0000000000000001 (non-biasable; age: 0)
3   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
3   0x00000000210df370 (thin lock: 0x00000000210df370)
3   0x0000000000000001 (non-biasable; age: 0)
4   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
4   0x00000000210df370 (thin lock: 0x00000000210df370)
4   0x0000000000000001 (non-biasable; age: 0)
5   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
5   0x00000000210df370 (thin lock: 0x00000000210df370)
5   0x0000000000000001 (non-biasable; age: 0)
6   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
6   0x00000000210df370 (thin lock: 0x00000000210df370)
6   0x0000000000000001 (non-biasable; age: 0)
7   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
7   0x00000000210df370 (thin lock: 0x00000000210df370)
7   0x0000000000000001 (non-biasable; age: 0)
8   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
8   0x00000000210df370 (thin lock: 0x00000000210df370)
8   0x0000000000000001 (non-biasable; age: 0)
9   0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
9   0x00000000210df370 (thin lock: 0x00000000210df370)
9   0x0000000000000001 (non-biasable; age: 0)
10  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
10  0x00000000210df370 (thin lock: 0x00000000210df370)
10  0x0000000000000001 (non-biasable; age: 0)
11  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
11  0x00000000210df370 (thin lock: 0x00000000210df370)
11  0x0000000000000001 (non-biasable; age: 0)
12  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
12  0x00000000210df370 (thin lock: 0x00000000210df370)
12  0x0000000000000001 (non-biasable; age: 0)
13  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
13  0x00000000210df370 (thin lock: 0x00000000210df370)
13  0x0000000000000001 (non-biasable; age: 0)
14  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
14  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
14  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
15  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
15  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
15  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
16  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
16  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
16  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
17  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
17  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
17  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
18  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
18  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
18  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
19  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
19  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
19  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
20  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
20  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
20  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
21  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
21  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
21  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
22  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
22  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
22  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
23  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
23  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
23  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
24  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
24  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
24  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
25  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
25  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
25  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
26  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
26  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
26  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
27  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
27  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
27  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
28  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
28  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
28  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
29  0x0000000020066805 (biased: 0x000000000008019a; epoch: 0; age: 0)
29  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)
29  0x0000000020067905 (biased: 0x000000000008019e; epoch: 0; age: 0)

===============>之前的30个对象，都是偏向于0x000000000008019e这个线程（t1）。
===============>之后开始进行了锁升级，升级成了轻量级锁（thin lock），释放后变成无锁状态（non-biasable）。
在经历14次锁升级，锁撤销后，发现第15次不在进行此类操作，而是直接修改偏向线程的id了。

因为锁升级和锁撤销会带来一定的性能损耗，所以jvm在这方面做了重偏向的优化。

2.4 批量锁撤销

当撤销偏向锁阈值超过 14 次后，jvm 会这样觉得，自己确实偏向错了，根本就不该偏向。于是整个类的所有对象都会变为不可偏向的，新建的对象也是不可偏向的。

使用三个线程，初始化一个50个对象的集合，t1处理数据，唤醒t2；t2处理完数据唤醒t3；代码如下：

    static Thread t1, t2, t3;

    public static void main(String[] args) throws InterruptedException {
        test3();
    }

    public static String print(String s) {
        String[] split = s.split("\r\n");
        return split[2].substring(41);
    }

    private static void test3() throws InterruptedException {
        Vector<Object> list = new Vector<>();
        t1 = new Thread(() -> {
            for (int i = 0; i < 50; i++) {
                Object d = new Object();
                list.add(d);
                synchronized (d) {
                    System.out.println(i + "\t" + print(ClassLayout.parseInstance(d).toPrintable()));
                }
            }
            LockSupport.unpark(t2);
        }, "t1");
        t1.start();

        t2 = new Thread(() -> {
            LockSupport.park();
            System.out.println("=========第二阶段======> ");
            for (int i = 0; i < 50; i++) {
                Object d = list.get(i);
                System.out.println("t2: " + i + "\t" + print(ClassLayout.parseInstance(d).toPrintable()));
                synchronized (d) {
                    System.out.println("t2: " + i + "\t" + print(ClassLayout.parseInstance(d).toPrintable()));
                }
                System.out.println("t2: " + i + "\t" + print(ClassLayout.parseInstance(d).toPrintable()));
            }
            LockSupport.unpark(t3);
        }, "t2");
        t2.start();


        t3 = new Thread(() -> {
            LockSupport.park();
            System.out.println("=========第三阶段======>");
            for (int i = 0; i < 50; i++) {
                Object d = list.get(i);
                System.out.println("t3: " + i + "\t" + print(ClassLayout.parseInstance(d).toPrintable()));
                synchronized (d) {
                    System.out.println("t3: " + i + "\t" + print(ClassLayout.parseInstance(d).toPrintable()));
                }
                System.out.println("t3: " + i + "\t" + print(ClassLayout.parseInstance(d).toPrintable()));
            }
        }, "t3");
        t3.start();

        t3.join();
        // 输出一个新的对象的对象头，发现是不可偏向锁
        System.out.println(print(ClassLayout.parseInstance(new Object()).toPrintable()));
    }

结果：

0   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
1   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
2   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
3   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
4   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
5   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
6   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
7   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
8   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
9   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
10  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
11  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
12  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
13  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
14  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
15  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
16  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
17  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
18  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
19  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
20  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
21  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
22  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
23  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
24  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
25  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
26  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
27  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
28  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
29  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
30  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
31  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
32  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
33  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
34  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
35  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
36  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
37  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
38  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
39  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
40  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
41  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
42  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
43  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
44  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
45  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
46  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
47  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
48  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
49  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
=========第二阶段======> 
t2: 0   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 0   0x000000002130f160 (thin lock: 0x000000002130f160)
t2: 0   0x0000000000000001 (non-biasable; age: 0)
t2: 1   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 1   0x000000002130f160 (thin lock: 0x000000002130f160)
t2: 1   0x0000000000000001 (non-biasable; age: 0)
t2: 2   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 2   0x000000002130f160 (thin lock: 0x000000002130f160)
t2: 2   0x0000000000000001 (non-biasable; age: 0)
t2: 3   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 3   0x000000002130f160 (thin lock: 0x000000002130f160)
t2: 3   0x0000000000000001 (non-biasable; age: 0)
t2: 4   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 4   0x000000002130f160 (thin lock: 0x000000002130f160)
t2: 4   0x0000000000000001 (non-biasable; age: 0)
t2: 5   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 5   0x000000002130f160 (thin lock: 0x000000002130f160)
t2: 5   0x0000000000000001 (non-biasable; age: 0)
t2: 6   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 6   0x000000002130f160 (thin lock: 0x000000002130f160)
t2: 6   0x0000000000000001 (non-biasable; age: 0)
t2: 7   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 7   0x000000002130f160 (thin lock: 0x000000002130f160)
t2: 7   0x0000000000000001 (non-biasable; age: 0)
t2: 8   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 8   0x000000002130f160 (thin lock: 0x000000002130f160)
t2: 8   0x0000000000000001 (non-biasable; age: 0)
t2: 9   0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 9   0x000000002130f160 (thin lock: 0x000000002130f160)
t2: 9   0x0000000000000001 (non-biasable; age: 0)
t2: 10  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 10  0x000000002130f160 (thin lock: 0x000000002130f160)
t2: 10  0x0000000000000001 (non-biasable; age: 0)
t2: 11  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 11  0x000000002130f160 (thin lock: 0x000000002130f160)
t2: 11  0x0000000000000001 (non-biasable; age: 0)
t2: 12  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 12  0x000000002130f160 (thin lock: 0x000000002130f160)
t2: 12  0x0000000000000001 (non-biasable; age: 0)
t2: 13  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 13  0x000000002130f160 (thin lock: 0x000000002130f160)
t2: 13  0x0000000000000001 (non-biasable; age: 0)
t2: 14  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 14  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 14  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 15  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 15  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 15  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 16  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 16  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 16  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 17  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 17  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 17  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 18  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 18  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 18  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 19  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 19  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 19  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 20  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 20  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 20  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 21  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 21  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 21  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 22  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 22  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 22  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 23  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 23  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 23  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 24  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 24  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 24  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 25  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 25  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 25  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 26  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 26  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 26  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 27  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 27  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 27  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 28  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 28  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 28  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 29  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 29  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 29  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 30  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 30  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 30  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 31  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 31  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 31  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 32  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 32  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 32  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 33  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 33  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 33  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 34  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 34  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 34  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 35  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 35  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 35  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 36  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 36  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 36  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 37  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 37  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 37  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 38  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 38  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 38  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 39  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 39  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 39  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 40  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 40  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 40  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 41  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 41  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 41  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 42  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 42  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 42  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 43  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 43  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 43  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 44  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 44  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 44  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 45  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 45  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 45  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 46  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 46  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 46  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 47  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 47  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 47  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 48  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 48  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 48  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 49  0x00000000202aa005 (biased: 0x0000000000080aa8; epoch: 0; age: 0)
t2: 49  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t2: 49  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
=========第三阶段======>
t3: 0   0x0000000000000001 (non-biasable; age: 0)
t3: 0   0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 0   0x0000000000000001 (non-biasable; age: 0)
t3: 1   0x0000000000000001 (non-biasable; age: 0)
t3: 1   0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 1   0x0000000000000001 (non-biasable; age: 0)
t3: 2   0x0000000000000001 (non-biasable; age: 0)
t3: 2   0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 2   0x0000000000000001 (non-biasable; age: 0)
t3: 3   0x0000000000000001 (non-biasable; age: 0)
t3: 3   0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 3   0x0000000000000001 (non-biasable; age: 0)
t3: 4   0x0000000000000001 (non-biasable; age: 0)
t3: 4   0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 4   0x0000000000000001 (non-biasable; age: 0)
t3: 5   0x0000000000000001 (non-biasable; age: 0)
t3: 5   0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 5   0x0000000000000001 (non-biasable; age: 0)
t3: 6   0x0000000000000001 (non-biasable; age: 0)
t3: 6   0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 6   0x0000000000000001 (non-biasable; age: 0)
t3: 7   0x0000000000000001 (non-biasable; age: 0)
t3: 7   0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 7   0x0000000000000001 (non-biasable; age: 0)
t3: 8   0x0000000000000001 (non-biasable; age: 0)
t3: 8   0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 8   0x0000000000000001 (non-biasable; age: 0)
t3: 9   0x0000000000000001 (non-biasable; age: 0)
t3: 9   0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 9   0x0000000000000001 (non-biasable; age: 0)
t3: 10  0x0000000000000001 (non-biasable; age: 0)
t3: 10  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 10  0x0000000000000001 (non-biasable; age: 0)
t3: 11  0x0000000000000001 (non-biasable; age: 0)
t3: 11  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 11  0x0000000000000001 (non-biasable; age: 0)
t3: 12  0x0000000000000001 (non-biasable; age: 0)
t3: 12  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 12  0x0000000000000001 (non-biasable; age: 0)
t3: 13  0x0000000000000001 (non-biasable; age: 0)
t3: 13  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 13  0x0000000000000001 (non-biasable; age: 0)
t3: 14  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 14  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 14  0x0000000000000001 (non-biasable; age: 0)
t3: 15  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 15  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 15  0x0000000000000001 (non-biasable; age: 0)
t3: 16  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 16  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 16  0x0000000000000001 (non-biasable; age: 0)
t3: 17  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 17  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 17  0x0000000000000001 (non-biasable; age: 0)
t3: 18  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 18  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 18  0x0000000000000001 (non-biasable; age: 0)
t3: 19  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 19  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 19  0x0000000000000001 (non-biasable; age: 0)
t3: 20  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 20  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 20  0x0000000000000001 (non-biasable; age: 0)
t3: 21  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 21  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 21  0x0000000000000001 (non-biasable; age: 0)
t3: 22  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 22  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 22  0x0000000000000001 (non-biasable; age: 0)
t3: 23  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 23  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 23  0x0000000000000001 (non-biasable; age: 0)
t3: 24  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 24  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 24  0x0000000000000001 (non-biasable; age: 0)
t3: 25  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 25  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 25  0x0000000000000001 (non-biasable; age: 0)
t3: 26  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 26  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 26  0x0000000000000001 (non-biasable; age: 0)
t3: 27  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 27  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 27  0x0000000000000001 (non-biasable; age: 0)
t3: 28  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 28  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 28  0x0000000000000001 (non-biasable; age: 0)
t3: 29  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 29  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 29  0x0000000000000001 (non-biasable; age: 0)
t3: 30  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 30  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 30  0x0000000000000001 (non-biasable; age: 0)
t3: 31  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 31  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 31  0x0000000000000001 (non-biasable; age: 0)
t3: 32  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 32  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 32  0x0000000000000001 (non-biasable; age: 0)
t3: 33  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 33  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 33  0x0000000000000001 (non-biasable; age: 0)
t3: 34  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 34  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 34  0x0000000000000001 (non-biasable; age: 0)
t3: 35  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 35  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 35  0x0000000000000001 (non-biasable; age: 0)
t3: 36  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 36  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 36  0x0000000000000001 (non-biasable; age: 0)
t3: 37  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 37  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 37  0x0000000000000001 (non-biasable; age: 0)
t3: 38  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 38  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 38  0x0000000000000001 (non-biasable; age: 0)
t3: 39  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 39  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 39  0x0000000000000001 (non-biasable; age: 0)
t3: 40  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 40  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 40  0x0000000000000001 (non-biasable; age: 0)
t3: 41  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 41  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 41  0x0000000000000001 (non-biasable; age: 0)
t3: 42  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 42  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 42  0x0000000000000001 (non-biasable; age: 0)
t3: 43  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 43  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 43  0x0000000000000001 (non-biasable; age: 0)
t3: 44  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 44  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 44  0x0000000000000001 (non-biasable; age: 0)
t3: 45  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 45  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 45  0x0000000000000001 (non-biasable; age: 0)
t3: 46  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 46  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 46  0x0000000000000001 (non-biasable; age: 0)
t3: 47  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 47  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 47  0x0000000000000001 (non-biasable; age: 0)
t3: 48  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 48  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 48  0x0000000000000001 (non-biasable; age: 0)
t3: 49  0x00000000202aa905 (biased: 0x0000000000080aaa; epoch: 0; age: 0)
t3: 49  0x000000002140f690 (thin lock: 0x000000002140f690)
t3: 49  0x0000000000000001 (non-biasable; age: 0)
0x0000000000000001 (non-biasable; age: 0)

第二阶段其实就是前一章节讲的重偏向优化
第三阶段开始，线程t3加入进来，前14个对象在第二阶段已经被撤销了偏向锁，所以这14个对象要不停的经历轻量级锁到锁撤销的状态。
当第三阶段达到第15次时，这之后的对象都是偏向于线程t2的，需要经历锁撤销，轻量级锁的过程，到结束并没有进行一个所偏向的优化了。
注意最后一行，当再次打印一个新创建的对象头时，发现是不可使用偏向锁的（关于这个阈值，有人说是40，我没有测试过）。

java并发编程（八）synchronized原理之偏向锁

一、偏向锁的状态

1.1 默认情况下的偏向锁

1.2 线程持有偏向锁

1.2.1 单线程持有

1.2.2 两个线程竞争

1.2.3 多个线程竞争

二、锁撤销

2.1 hashcode

2.2 偏向锁锁撤销 - 升级轻量级锁或重量级锁

2.3 重偏向优化

2.4 批量锁撤销

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