目录
- Object类自带的方法
- Condition接口
- LockSupport
Object类自带的方法
使用wait()方法来阻塞线程,使用notify()和notifyAll()方法来唤醒线程。 调用wait()方法后,线程将被阻塞,wait()方法将会释放当前持有的监视器锁(monitor),直到有线程调用notify/notifyAll()方法后方能继续执行。 notify/notifyAll()方法只是解除了等待线程的阻塞,并不会马上释放监视器锁,而是在相应的被synchronized关键字修饰的同步方法或同步代码块执行结束后才自动释放锁。 默认使用非公平锁,无法修改。
缺点:
- 使用几个方法时,必须处于被synchronized关键字修饰的同步方法或同步代码块中,否则程序运行时,会抛出IllegalMonitorStateException异常。
- 线程的唤醒必须在线程阻塞之后,否则,当前线程被阻塞之后,一直没有唤醒,线程将会一直等待下去(对比LockSupport)
public class SynchronizedDemo { // 三个线程交替打印ABC public static void main(String[] args) { Print print = new Print(); new Thread(() -> { while (true) { print.printA(); } }, "A").start(); new Thread(() -> { while (true) { print.printB(); } }, "B").start(); new Thread(() -> { while (true) { print.printC(); } }, "C").start(android); } } class Print { Object object = new Object(); int num = 1; public void printA() { synchronized (object) { try { while (num != 1) { object.wait(); } for (int i = 0; i < 5; i++) { System.out.println(Thread.currentThread().getName() + "==>A"); } num = 2; object.notifyAll(); } catch (InterruptedException e) { e.printStackTrace(); } } } public void printB() { synchronized (object) { try { while (num != 2) { object.wait(); } for (int i = 0; i < 10; i++) { System.ou编程客栈t.println(Thread.currentThread().getName() + "==>B"); } num = 3; object.notifyAll(); } catch (InterruptedException e) { e.printStackTr编程客栈ace(); } } } public void printC() { synchronized (object) { try { while (num != 3) { object.wait(); } for (int i = 0; i < 15; i++) { System.out.println(Thread.currentThread().getName() + "==>C"); } num = 1; www.devze.com object.notifyAll(); } catch (InterruptedException e) { e.printStackTrace(); } } } }
Cohttp://www.devze.comndition接口
使用await()方法来阻塞线程,signal()/singnalAll()方法来唤醒线程。
需要使用lock对象的newCondition()方法获得Condition条件对象(可有多个)。
可实现公平锁,默认是非公平锁
缺点:
- 必须被Lock包裹,否则会在运行时抛出IllegalMonitorStateException异常。
- 线程的唤醒必须在线程阻塞之后
- Lock的实现是基于AQS,效率稍高于synchronized
public class ConditionDemo { // 三个线程交替打印ABC public static void main(String[] args) { Print print = new Print(); new Thread(() -> { while (true) { print.printA(); } }, "A").start(); new Thread(() -> { while (true) { print.printB(); } }, "B").start(); new Thread(() -> { while (true) { print.printC(); } }, "C").start(); } } class Print { private Lock lock = new ReentrantLock(); private Condition condition1 = lock.newCondition(); private Condition condition2 = lock.newCondition(); private Condition condition3 = lock.newCondition(); private int num = 1; public void printA() { lock.lock(); try { while (num != 1) { condition1.await(); } for (int i = 0; i < 5; ++i) { System.out.println(Thread.currentThread().getName() + "==>A"); } num = 2; condition2.signal(); } catch (InterruptedException e) { e.printStackTrace(); } finally { lock.unlock(); } } public void printB() { lock.lock(); try { while (num != 2) { condition2.await(); } for (int i = 0; i < 10; ++i) { System.out.println(Thread.currentThread().getName() + "==>B"); } num = 3; condition3.signal(); } catch (InterruptedException e) { e.printStackTrace(); } finally { lock.unlock(); } } public void printC() { lock.lock(); try { while (num != 3) { condition3.await(); } for (int i = 0; i < 15; ++i) { System.out.println(Thread.currentThread().getName() + "==>C"); } num = 1; condition1.signal(); } catch (InterruptedException e) { e.printStackTrace(); } finally { lock.unlock(); } } }
LockSupport
使用park()来阻塞线程,用unpark()方法来唤醒线程。 这里有一个许可证的概念,许可不能累积,并且最多只能有一个许可,只有1和0的区别。
特点:
- 使用灵活,可以直接使用
- 线程唤醒可在线程阻塞之前,因为调用unpark()方法后,线程已经获得了一个许可证(但也只能有一个许可证),之后阻塞时,可以直接使用这个许可证来通行。
- 效率高
public class LockSupportDemo { // 三个线程交替打印ABC public static void main(String[] args) throws Exception { Print print = new Print(); Thread threadA = new Thread(() -> { while (true) { print.printA(); } }, "A"); Thread threadB = new Thread(() -> { while (true) { print.printB(); } }, "B"); Thread threadC = new Thread(() -> { while (true) { print.printC(); } }, "C"); threadA.start(); threadB.start(); threadC.start(); while (true) { LockSupport.unpark(threadA); LockSupport.unpark(threadB); LockSupport.unpark(threadC); } } } class Print { private int num = 1; public void printA() { while (num != 1) { LockSupport.park(); } for (int i = 0; i < 5; i++) { System.out.println(Thread.currentThread().getName() + "==>A"); } num = 2; } public void printB() { while (num != 2) { LockSupport.park(); } for (int i = 0; i < 10; i++) { System.out.println(Thread.currentThread().getName() + "==>B"); } num = 3; } public void printC() { while (num != 3) { LockSupport.park(); } for (int i = 0; i < 15; i++) { System.out.println(Thread.currentThread().getName() + "==>C"); } num = 1; } }
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