[TOC]
1.不正确的访问资源
懒汉单例模式,线程不安全
public class SingletonDemo1 {
private static SingletonDemo1 instance;
private SingletonDemo1(){}
public static SingletonDemo1 getInstance(){
if (instance == null) {
instance = new SingletonDemo1();
}
return instance;
}
}
2.锁(synchronized)
public class SingletonDemo2 {
private static SingletonDemo2 instance;
private SingletonDemo2(){}
public static synchronized SingletonDemo2 getInstance(){
if (instance == null) {
instance = new SingletonDemo2();
}
return instance;
}
}
3.显式的锁(Lock)
import java.util.concurrent.locks.*;
public class SingletonDemo2 {
private static SingletonDemo2 instance;
private SingletonDemo2(){}
private static Lock lock = new ReentrantLock();
public static SingletonDemo2 getInstance(){
lock.lock();
try{
if (instance == null) {
instance = new SingletonDemo2();
}
return instance;
} finally {
lock.unlock();
}
}
}
用synchronized不能尝试着获取锁且最终获取锁会失败,或尝试获取锁一段时间,然后放弃它时,应使用Lock锁:
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.*;
public class SingletonDemo2 {
private static SingletonDemo2 instance;
private SingletonDemo2(){}
private static Lock lock = new ReentrantLock();
public static SingletonDemo2 getInstance(){
boolean captured = false;
try {
captured = lock.tryLock(2, TimeUnit.SECONDS);
} catch (InterruptedException e) {
e.printStackTrace();
}
lock.lock();
try{
if (instance == null) {
instance = new SingletonDemo2();
}
return instance;
} finally {
if (captured){
lock.unlock();
}
}
}
}
4.原子性
原子性操作是不能被线程调度机制中断的操作。原子性可应用于除long 和 double 之外的所有基本数据类型,因jvm会将64位的long和double的读取和写入当做两个分离的32位操作,从而产生“字撕裂”。如果使用volatile关键字,可以使long和double 获得原子性。
java 中仅基本数据类型(除long和double)的读取和赋值为院子操作。
public class Atomicity {
int i;
void f1() { i++; }
void f2() { i += 3; }
} /* Output: (Sample)
...
void f1();
Code:
0: aload_0
1: dup
2: getfield #2; //Field i:I
5: iconst_1
6: iadd
7: putfield #2; //Field i:I
10: return
void f2();
Code:
0: aload_0
1: dup
2: getfield #2; //Field i:I
5: iconst_3
6: iadd
7: putfield #2; //Field i:I
10: return
*///:~
每条指令都会产生一个get和put,它们之间还有其他指令。
5.原子类
AtomicInteger
AtomicLong
AtomicReference
6.临界区
synchronized被用来指定某个对象,此对象的锁被用来对花括号内的代码进行同步控制:
synchronized(syncObject){
statements;
}
7.线程本地存储 (ThreadLocal)
线程本地存储是一种自动化的机制,可以为使用相同变量的每个线程都创建不同的存储。
//: ThreadLocalVariableHolder.java
// Automatically giving each thread its own storage.
import java.util.concurrent.*;
import java.util.*;
class Accessor implements Runnable {
private final int id;
public Accessor(int idn) { id = idn; }
public void run() {
while(!Thread.currentThread().isInterrupted()) {
ThreadLocalVariableHolder.increment();
System.out.println(this);
Thread.yield();
}
}
public String toString() {
return "#" + id + ": " +
ThreadLocalVariableHolder.get();
}
}
public class ThreadLocalVariableHolder {
private static ThreadLocal<Integer> value =
new ThreadLocal<Integer>() {
private Random rand = new Random(47);
protected synchronized Integer initialValue() {
return rand.nextInt(10000);
}
};
public static void increment() {
value.set(value.get() + 1);
}
public static int get() { return value.get(); }
public static void main(String[] args) throws Exception {
ExecutorService exec = Executors.newCachedThreadPool();
for(int i = 0; i < 5; i++)
exec.execute(new Accessor(i));
TimeUnit.SECONDS.sleep(3); // Run for a while
exec.shutdownNow(); // All Accessors will quit
}
} /* Output: (Sample)
#0: 9259
#1: 556
#2: 6694
#3: 1862
#4: 962
#0: 9260
#1: 557
#2: 6695
#3: 1863
#4: 963
...
*///:~
8.在阻塞时终结(中断)
线程状态:
- 新建
- 就绪
- 阻塞
- 死亡
进入阻塞状态的原因:
- 调用sleep()使任务休眠
- 调用wait()使任务挂起
- 任务在等待某个输入/输出完成
- 任务在等锁
中断任务可调用Thread.interrupted()或 调用executor的shutdownNow()方法,会发送一个interrupted()调用给他启动的所有线程。如果使用Executor的submit()来启动任务,就可以持有该任务的上下文。submit()将返回一个Future<?>,可以调用其cancel()来只中断某个特定的任务。
其中仅sleep状态可中断。等待io和等待锁,都会使任务的中断失效:
解决:
1.关闭任务在其上发生阻塞的底层资源。
2.interrupt()可打断被互斥阻塞的调用
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