前言：
线程开始运行，拥有自己的栈空间，就如同一个脚本一样，按照既定的代码一步一步地执行，直到终止。但是，每个运行中的线程，如果仅仅是孤立地运行，那么没有一点儿价值，或者说价值很少，如果多个线程能够相互配合完成工作，包括数据之间的共享，协同处理事情。这将会带来巨大的价值。
1、线程间的共享
Java支持多个线程同时访问一个对象或者对象的成员变量，关键字synchronized可以修饰方法或者以同步块的形式来进行使用，它主要确保多个线程在同一个时刻，只能有一个线程处于方法或者同步块中，它保证了线程对变量访问的可见性和排他性，又称为内置锁机制。
对象锁和类锁：
对象锁：是用于对象实例方法，或者一个对象实例上的。
类锁：是用于类的静态方法或者一个类的class对象上的。
我们知道，类的对象实例可以有很多个，但是每个类只有一个class对象，所以不同对象实例的对象锁是互不干扰的，但是每个类只有一个类锁。
但是有一点必须注意的是，其实类锁只是一个概念上的东西，并不是真实存在的，类锁其实锁的是每个类的对应的class对象。类锁和对象锁之间也是互不干扰的。
2、线程间的协作
线程之间相互配合，完成某项工作，比如：一个线程修改了一个对象的值，而另一个线程感知到了变化，然后进行相应的操作，整个过程开始于一个线程，而最终执行又是另一个线程。前者是生产者，后者就是消费者，这种模式隔离了“做什么”（what）和“怎么做”（How），简单的办法是让消费者线程不断地循环检查变量是否符合预期在while循环中设置不满足的条件，如果条件满足则退出while循环，从而完成消费者的工作。却存在如下问题：
1） 难以确保及时性。
2）难以降低开销。如果降低睡眠的时间，比如休眠1毫秒，这样消费者能更加迅速地发现条件变化，但是却可能消耗更多的处理器资源，造成了无端的浪费。
等待/通知机制
万物皆对象，所有的对象 extends Object,那么在Object中存在wait()、notify()、notifyAll()就很好解决了上述问题
/**
* Wakes up a single thread that is waiting on this object's
* monitor. If any threads are waiting on this object, one of them
* is chosen to be awakened. The choice is arbitrary and occurs at
* the discretion of the implementation. A thread waits on an object's
* monitor by calling one of the {@code wait} methods.
* <p>
* The awakened thread will not be able to proceed until the current
* thread relinquishes the lock on this object. The awakened thread will
* compete in the usual manner with any other threads that might be
* actively competing to synchronize on this object; for example, the
* awakened thread enjoys no reliable privilege or disadvantage in being
* the next thread to lock this object.
* <p>
* This method should only be called by a thread that is the owner
* of this object's monitor. A thread becomes the owner of the
* object's monitor in one of three ways:
* <ul>
* <li>By executing a synchronized instance method of that object.
* <li>By executing the body of a {@code synchronized} statement
* that synchronizes on the object.
* <li>For objects of type {@code Class,} by executing a
* synchronized static method of that class.
* </ul>
* <p>
* Only one thread at a time can own an object's monitor.
*
* @throws IllegalMonitorStateException if the current thread is not
* the owner of this object's monitor.
* @see java.lang.Object#notifyAll()
* @see java.lang.Object#wait()
*/
@FastNative
public final native void notify();
/**
* Wakes up all threads that are waiting on this object's monitor. A
* thread waits on an object's monitor by calling one of the
* {@code wait} methods.
* <p>
* The awakened threads will not be able to proceed until the current
* thread relinquishes the lock on this object. The awakened threads
* will compete in the usual manner with any other threads that might
* be actively competing to synchronize on this object; for example,
* the awakened threads enjoy no reliable privilege or disadvantage in
* being the next thread to lock this object.
* <p>
* This method should only be called by a thread that is the owner
* of this object's monitor. See the {@code notify} method for a
* description of the ways in which a thread can become the owner of
* a monitor.
*
* @throws IllegalMonitorStateException if the current thread is not
* the owner of this object's monitor.
* @see java.lang.Object#notify()
* @see java.lang.Object#wait()
*/
@FastNative
public final native void notifyAll();
/**
* Causes the current thread to wait until either another thread invokes the
* {@link java.lang.Object#notify()} method or the
* {@link java.lang.Object#notifyAll()} method for this object, or a
* specified amount of time has elapsed.
* <p>
* The current thread must own this object's monitor.
* <p>
* This method causes the current thread (call it <var>T</var>) to
* place itself in the wait set for this object and then to relinquish
* any and all synchronization claims on this object. Thread <var>T</var>
* becomes disabled for thread scheduling purposes and lies dormant
* until one of four things happens:
* <ul>
* <li>Some other thread invokes the {@code notify} method for this
* object and thread <var>T</var> happens to be arbitrarily chosen as
* the thread to be awakened.
* <li>Some other thread invokes the {@code notifyAll} method for this
* object.
* <li>Some other thread {@linkplain Thread#interrupt() interrupts}
* thread <var>T</var>.
* <li>The specified amount of real time has elapsed, more or less. If
* {@code timeout} is zero, however, then real time is not taken into
* consideration and the thread simply waits until notified.
* </ul>
* The thread <var>T</var> is then removed from the wait set for this
* object and re-enabled for thread scheduling. It then competes in the
* usual manner with other threads for the right to synchronize on the
* object; once it has gained control of the object, all its
* synchronization claims on the object are restored to the status quo
* ante - that is, to the situation as of the time that the {@code wait}
* method was invoked. Thread <var>T</var> then returns from the
* invocation of the {@code wait} method. Thus, on return from the
* {@code wait} method, the synchronization state of the object and of
* thread {@code T} is exactly as it was when the {@code wait} method
* was invoked.
* <p>
* A thread can also wake up without being notified, interrupted, or
* timing out, a so-called <i>spurious wakeup</i>. While this will rarely
* occur in practice, applications must guard against it by testing for
* the condition that should have caused the thread to be awakened, and
* continuing to wait if the condition is not satisfied. In other words,
* waits should always occur in loops, like this one:
* <pre>
* synchronized (obj) {
* while (<condition does not hold>)
* obj.wait(timeout);
* ... // Perform action appropriate to condition
* }
* </pre>
* (For more information on this topic, see Section 3.2.3 in Doug Lea's
* "Concurrent Programming in Java (Second Edition)" (Addison-Wesley,
* 2000), or Item 50 in Joshua Bloch's "Effective Java Programming
* Language Guide" (Addison-Wesley, 2001).
*
* <p>If the current thread is {@linkplain java.lang.Thread#interrupt()
* interrupted} by any thread before or while it is waiting, then an
* {@code InterruptedException} is thrown. This exception is not
* thrown until the lock status of this object has been restored as
* described above.
*
* <p>
* Note that the {@code wait} method, as it places the current thread
* into the wait set for this object, unlocks only this object; any
* other objects on which the current thread may be synchronized remain
* locked while the thread waits.
* <p>
* This method should only be called by a thread that is the owner
* of this object's monitor. See the {@code notify} method for a
* description of the ways in which a thread can become the owner of
* a monitor.
*
* @param millis the maximum time to wait in milliseconds.
* @throws IllegalArgumentException if the value of timeout is
* negative.
* @throws IllegalMonitorStateException if the current thread is not
* the owner of the object's monitor.
* @throws InterruptedException if any thread interrupted the
* current thread before or while the current thread
* was waiting for a notification. The <I>interrupted
* status</i> of the current thread is cleared when
* this exception is thrown.
* @see java.lang.Object#notify()
* @see java.lang.Object#notifyAll()
*/
public final void wait(long millis) throws InterruptedException {
wait(millis, 0);
}
/**
* Causes the current thread to wait until another thread invokes the
* {@link java.lang.Object#notify()} method or the
* {@link java.lang.Object#notifyAll()} method for this object, or
* some other thread interrupts the current thread, or a certain
* amount of real time has elapsed.
* <p>
* This method is similar to the {@code wait} method of one
* argument, but it allows finer control over the amount of time to
* wait for a notification before giving up. The amount of real time,
* measured in nanoseconds, is given by:
* <blockquote>
* <pre>
* 1000000*timeout+nanos</pre></blockquote>
* <p>
* In all other respects, this method does the same thing as the
* method {@link #wait(long)} of one argument. In particular,
* {@code wait(0, 0)} means the same thing as {@code wait(0)}.
* <p>
* The current thread must own this object's monitor. The thread
* releases ownership of this monitor and waits until either of the
* following two conditions has occurred:
* <ul>
* <li>Another thread notifies threads waiting on this object's monitor
* to wake up either through a call to the {@code notify} method
* or the {@code notifyAll} method.
* <li>The timeout period, specified by {@code timeout}
* milliseconds plus {@code nanos} nanoseconds arguments, has
* elapsed.
* </ul>
* <p>
* The thread then waits until it can re-obtain ownership of the
* monitor and resumes execution.
* <p>
* As in the one argument version, interrupts and spurious wakeups are
* possible, and this method should always be used in a loop:
* <pre>
* synchronized (obj) {
* while (<condition does not hold>)
* obj.wait(timeout, nanos);
* ... // Perform action appropriate to condition
* }
* </pre>
* This method should only be called by a thread that is the owner
* of this object's monitor. See the {@code notify} method for a
* description of the ways in which a thread can become the owner of
* a monitor.
*
* @param millis the maximum time to wait in milliseconds.
* @param nanos additional time, in nanoseconds range
* 0-999999.
* @throws IllegalArgumentException if the value of timeout is
* negative or the value of nanos is
* not in the range 0-999999.
* @throws IllegalMonitorStateException if the current thread is not
* the owner of this object's monitor.
* @throws InterruptedException if any thread interrupted the
* current thread before or while the current thread
* was waiting for a notification. The <I>interrupted
* status</i> of the current thread is cleared when
* this exception is thrown.
*/
@FastNative
public final native void wait(long millis, int nanos) throws InterruptedException;
/**
* Causes the current thread to wait until another thread invokes the
* {@link java.lang.Object#notify()} method or the
* {@link java.lang.Object#notifyAll()} method for this object.
* In other words, this method behaves exactly as if it simply
* performs the call {@code wait(0)}.
* <p>
* The current thread must own this object's monitor. The thread
* releases ownership of this monitor and waits until another thread
* notifies threads waiting on this object's monitor to wake up
* either through a call to the {@code notify} method or the
* {@code notifyAll} method. The thread then waits until it can
* re-obtain ownership of the monitor and resumes execution.
* <p>
* As in the one argument version, interrupts and spurious wakeups are
* possible, and this method should always be used in a loop:
* <pre>
* synchronized (obj) {
* while (<condition does not hold>)
* obj.wait();
* ... // Perform action appropriate to condition
* }
* </pre>
* This method should only be called by a thread that is the owner
* of this object's monitor. See the {@code notify} method for a
* description of the ways in which a thread can become the owner of
* a monitor.
*
* @throws IllegalMonitorStateException if the current thread is not
* the owner of the object's monitor.
* @throws InterruptedException if any thread interrupted the
* current thread before or while the current thread
* was waiting for a notification. The <I>interrupted
* status</i> of the current thread is cleared when
* this exception is thrown.
* @see java.lang.Object#notify()
* @see java.lang.Object#notifyAll()
*/
@FastNative
public final native void wait() throws InterruptedException;
是指一个线程A调用了对象O的wait()方法进入等待状态，而另一个线程B调用了对象O的notify()或者notifyAll()方法，线程A收到通知后从对象O的wait()方法返回，进而执行后续操作。上述两个线程通过对象O来完成交互，而对象上的wait()和notify/notifyAll()的关系就如同开关信号一样，用来完成等待方和通知方之间的交互工作。
notify()：
通知一个在对象上等待的线程,使其从wait方法返回,而返回的前提是该线程获取到了对象的锁，没有获得锁的线程重新进入WAITING状态。
notifyAll()：
通知所有等待在该对象上的线程
wait():
调用该方法的线程进入 WAITING状态,只有等待另外线程的通知或被中断才会返回.需要注意,调用wait()方法后,会释放对象的锁
wait(long)
超时等待一段时间,这里的参数时间是毫秒,也就是等待长达n毫秒,如果没有通知就超时返回
wait (long,int)
对于超时时间更细粒度的控制,可以达到纳秒
等待和通知的标准范式:
等待方遵循如下原则:
1）获取对象的锁。
2）如果条件不满足，那么调用对象的wait()方法，被通知后仍要检查条件。
3）条件满足则执行对应的逻辑。
image.gif
通知方遵循如下原则:
1）获得对象的锁。
2）改变条件。
3）通知所有等待在对象上的线程。
image.gif
在调用wait()之前，线程必须要获得该对象的对象级别锁，即只能在同步方法或同步块中调用wait()方法，进入wait()方法后，当前线程释放锁，在从wait()返回前，线程与其他线程竞争重新获得锁，notifyAll方法一旦该对象锁被释放（notifyAll线程退出调用了notifyAll的synchronized代码块的时候），他们就会去竞争。如果其中一个线程获得了该对象锁，它就会继续往下执行，在它退出synchronized代码块，释放锁后，其他的已经被唤醒的线程将会继续竞争获取该锁，一直进行下去，直到所有被唤醒的线程都执行完毕。
notify和notifyAll应该用谁?
notify()：只是唤醒在该对象上其中一个线程，但是并不知道是哪个线程。
notifyAll()：唤醒的是该对象上的所有线程，在多个线程同时作用到一个对象上的时候，最好使用notifyAll()唤醒所有的线程。
3、ThreadLocal
ThreadLocal，即线程变量，是一个以ThreadLocal对象为键、任意对象为值的存储结构。这个结构被附带在线程上，也就是说一个线程可以根据一个ThreadLocal对象查询到绑定在这个线程上的一个值, ThreadLocal往往用来实现变量在线程之间的隔离。
ThreadLocal类接口很简单，只有4个方法，我们先来了解一下：
• void set(Object value)
设置当前线程的线程局部变量的值。
• public Object get()
该方法返回当前线程所对应的线程局部变量。
• public void remove()
将当前线程局部变量的值删除，目的是为了减少内存的占用，该方法是JDK 5.0新增的方法。需要指出的是，当线程结束后，对应该线程的局部变量将自动被垃圾回收，所以显式调用该方法清除线程的局部变量并不是必须的操作，但它可以加快内存回收的速度。
• protected Object initialValue()
返回该线程局部变量的初始值，该方法是一个protected的方法，显然是为了让子类覆盖而设计的。这个方法是一个延迟调用方法，在线程第1次调用get()或set(Object)时才执行，并且仅执行1次。ThreadLocal中的缺省实现直接返回一个null。
public final static ThreadLocal<String> RESOURCE = new ThreadLocal<String>();RESOURCE代表一个能够存放String类型的ThreadLocal对象。此时不论什么一个线程能够并发访问这个变量，对它进行写入、读取操作，都是线程安全的。
4、显式锁-Lock
我们一般的Java程序是靠synchronized关键字实现锁功能的，使用synchronized关键字将会隐式地获取锁，但是它将锁的获取和释放固化了，也就是先获取再释放。synchronized属于Java语言层面的锁，也被称之为内置锁。synchronized这种机制，一旦开始获取锁，是不能中断的，也不提供尝试获取锁的机制。而Lock是由Java在语法层面提供的，锁的获取和释放需要我们明显的去获取，因此被称为显式锁。并且提供了synchronized不提供的机制。
image.gif
Lock接口和核心方法
在finally块中释放锁，目的是保证在获取到锁之后，最终能够被释放。
image.gif
image.gif
可重入锁ReentrantLock、所谓锁的公平和非公平
而synchronized关键字隐式的支持重进入，比如一个synchronized修饰的递归方法，在方法执行时，执行线程在获取了锁之后仍能连续多次地获得该锁。ReentrantLock在调用lock()方法时，已经获取到锁的线程，能够再次调用lock()方法获取锁而不被阻塞。
公平和非公平锁
如果在时间上，先对锁进行获取的请求一定先被满足，那么这个锁是公平的，反之，是不公平的。公平的获取锁，也就是等待时间最长的线程最优先获取锁，也可以说锁获取是顺序的。
ReentrantLock提供了一个构造函数，能够控制锁是否是公平的。事实上，公平的锁机制往往没有非公平的效率高。原因是，在恢复一个被挂起的线程与该线程真正开始运行之间存在着严重的延迟。假设线程A持有一个锁,并且线程B请求这个锁。由于这个锁已被线程A持有,因此B将被挂起。当A释放锁时,B将被唤醒,因此会再次尝试获取锁。与此同时,如果C也请求这个锁,那么C很可能会在B被完全唤醒之前获得、使用以及释放这个锁。这样的情况是一种“双赢”的局面:B获得锁的时刻并没有推迟,C更早地获得了锁,并且吞吐量也获得了提高。
读写锁ReentrantReadWriteLock
之前提到锁（synchronized和ReentrantLock）基本都是排他锁，这些锁在同一时刻只允许一个线程进行访问，而读写锁在同一时刻可以允许多个读线程访问，但是在写线程访问时，所有的读线程和其他写线程均被阻塞。读写锁维护了一对锁，一个读锁和一个写锁，通过分离读锁和写锁，使得并发性相比一般的排他锁有了很大提升。除了保证写操作对读操作的可见性以及并发性的提升之外，读写锁能够简化读写交互场景的编程方式。假设在程序中定义一个共享的用作缓存数据结构，它大部分时间提供读服务（例如查询和搜索），而写操作占有的时间很少，但是写操作完成之后的更新需要对后续的读服务可见。一般情况下，读写锁的性能都会比排它锁好，因为大多数场景读是多于写的。在读多于写的情况下，读写锁能够提供比排它锁更好的并发性和吞吐量
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