Java:使用synchronized和Lock对象获取对象锁

在并发环境下，解决共享资源冲突问题时，可以考虑使用锁机制。

1.对象的锁

所有对象都自动含有单一的锁。

JVM负责跟踪对象被加锁的次数。如果一个对象被解锁，其计数变为0。在任务（线程）第一次给对象加锁的时候，计数变为1。每当这个相同的任务（线程）在此对象上获得锁时，计数会递增。

只有首先获得锁的任务（线程）才能继续获取该对象上的多个锁。

每当任务离开一个synchronized方法，计数递减，当计数为0的时候，锁被完全释放，此时别的任务就可以使用此资源。

2.synchronized同步块

2.1同步到单一对象锁

当使用同步块时，如果方法下的同步块都同步到一个对象上的锁，则所有的任务（线程）只能互斥的进入这些同步块。

Resource1.java演示了三个线程（包括main线程）试图进入某个类的三个不同的方法的同步块中，虽然这些同步块处在不同的方法中，但由于是同步到同一个对象（当前对象 synchronized (this)），所以对它们的方法依然是互斥的。

Resource1.java

package com.zj.lock;

import java.util.concurrent.TimeUnit;

 

public class Resource1 {

    public void f() {

       // other operations should not be locked...

       System.out.println(Thread.currentThread().getName()

              + ":not synchronized in f()");

       synchronized (this) {

           for (int i = 0; i < 5; i++) {

              System.out.println(Thread.currentThread().getName()

                     + ":synchronized in f()");

              try {

                  TimeUnit.SECONDS.sleep(3);

              } catch (InterruptedException e) {

                  e.printStackTrace();

              }

           }

       }

    }

 

    public void g() {

       // other operations should not be locked...

       System.out.println(Thread.currentThread().getName()

              + ":not synchronized in g()");

       synchronized (this) {

           for (int i = 0; i < 5; i++) {

              System.out.println(Thread.currentThread().getName()

                     + ":synchronized in g()");

              try {

                  TimeUnit.SECONDS.sleep(3);

              } catch (InterruptedException e) {

                  e.printStackTrace();

              }

           }

       }

    }

 

    public void h() {

       // other operations should not be locked...

       System.out.println(Thread.currentThread().getName()

              + ":not synchronized in h()");

       synchronized (this) {

           for (int i = 0; i < 5; i++) {

              System.out.println(Thread.currentThread().getName()

                     + ":synchronized in h()");

              try {

                  TimeUnit.SECONDS.sleep(3);

              } catch (InterruptedException e) {

                  e.printStackTrace();

              }

           }

       }

    }

 

    public static void main(String[] args) {

       final Resource1 rs = new Resource1();

 

       new Thread() {

           public void run() {

              rs.f();

           }

       }.start();

 

       new Thread() {

           public void run() {

              rs.g();

           }

       }.start();

 

       rs.h();

    }

}

结果：

Thread-0:not synchronized in f()

Thread-0:synchronized in f()

main:not synchronized in h()

Thread-1:not synchronized in g()

Thread-0:synchronized in f()

Thread-0:synchronized in f()

Thread-0:synchronized in f()

Thread-0:synchronized in f()

Thread-1:synchronized in g()

Thread-1:synchronized in g()

Thread-1:synchronized in g()

Thread-1:synchronized in g()

Thread-1:synchronized in g()

main:synchronized in h()

main:synchronized in h()

main:synchronized in h()

main:synchronized in h()

main:synchronized in h()

2.2 同步到多个对象锁

Resource1.java演示了三个线程（包括main线程）试图进入某个类的三个不同的方法的同步块中，这些同步块处在不同的方法中，并且是同步到三个不同的对象（synchronized (this)，synchronized (syncObject1)，synchronized (syncObject2)），所以对它们的方法中的临界资源访问是独立的。

Resource2.java

package com.zj.lock;

import java.util.concurrent.TimeUnit;

 

public class Resource2 {

    private Object syncObject1 = new Object();

    private Object syncObject2 = new Object();

 

    public void f() {

       // other operations should not be locked...

       System.out.println(Thread.currentThread().getName()

              + ":not synchronized in f()");

       synchronized (this) {

           for (int i = 0; i < 5; i++) {

              System.out.println(Thread.currentThread().getName()

                     + ":synchronized in f()");

              try {

                  TimeUnit.SECONDS.sleep(3);

              } catch (InterruptedException e) {

                  e.printStackTrace();

              }

           }

       }

    }

 

    public void g() {

       // other operations should not be locked...

       System.out.println(Thread.currentThread().getName()

              + ":not synchronized in g()");

       synchronized (syncObject1) {

           for (int i = 0; i < 5; i++) {

              System.out.println(Thread.currentThread().getName()

                     + ":synchronized in g()");

              try {

                  TimeUnit.SECONDS.sleep(3);

              } catch (InterruptedException e) {

                  e.printStackTrace();

              }

           }

       }

    }

 

    public void h() {

       // other operations should not be locked...

       System.out.println(Thread.currentThread().getName()

              + ":not synchronized in h()");

       synchronized (syncObject2) {

           for (int i = 0; i < 5; i++) {

              System.out.println(Thread.currentThread().getName()

                     + ":synchronized in h()");

              try {

                  TimeUnit.SECONDS.sleep(3);

              } catch (InterruptedException e) {

                  e.printStackTrace();

              }

           }

       }

    }

 

    public static void main(String[] args) {

       final Resource2 rs = new Resource2();

 

       new Thread() {

           public void run() {

              rs.f();

           }

       }.start();

 

       new Thread() {

           public void run() {

              rs.g();

           }

       }.start();

 

       rs.h();

    }

}

结果：

Thread-0:not synchronized in f()

Thread-0:synchronized in f()

main:not synchronized in h()

main:synchronized in h()

Thread-1:not synchronized in g()

Thread-1:synchronized in g()

Thread-0:synchronized in f()

main:synchronized in h()

Thread-1:synchronized in g()

Thread-0:synchronized in f()

main:synchronized in h()

Thread-1:synchronized in g()

Thread-0:synchronized in f()

main:synchronized in h()

Thread-1:synchronized in g()

Thread-0:synchronized in f()

main:synchronized in h()

Thread-1:synchronized in g()