先看测试方法：

package com.vincent;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;/** * Vincent 创建于 2016/5/11. */public class Main {    public static void main(String[] args) {        ExecutorService threadPool = Executors.newCachedThreadPool();        final SynchronizedDemo synchronizedDemo=new SynchronizedDemo();        for (int i=0;i<5;i++){            threadPool.execute(new Runnable() {                public void run() {                    synchronizedDemo.b();                }            });        }        for (int i=0;i<3;i++){            threadPool.execute(new Runnable() {                public void run() {                    synchronizedDemo.a();                }            });        }    }}

• 多个线程同时访问多个synchronized方法：

package com.vincent;import java.util.concurrent.TimeUnit;/** * Vincent 创建于 2016/5/11. */public class SynchronizedDemo {    private Object lock1 = new Object();    private Object lock2 = new Object();    long currentTimeMillis = System.currentTimeMillis();    public synchronized void a() {        long num = System.currentTimeMillis() - currentTimeMillis;        System.out.println("a.num=" + num);        try {            TimeUnit.SECONDS.sleep(2);        } catch (InterruptedException e) {            e.printStackTrace();        }    }    public synchronized void b() {        long num = System.currentTimeMillis() - currentTimeMillis;        System.out.println("b.num=" + num);        try {            TimeUnit.SECONDS.sleep(1);        } catch (InterruptedException e) {            e.printStackTrace();        }    }}

输出如下：

b.num=2a.num=1003a.num=3003a.num=5004b.num=7004b.num=8004b.num=9004b.num=10004

由此可以看出：多线程在同一时刻只能有一个线程访问对象的synchronized方法

• 多线程同时访问一个synchronized方法，一个非synchronized方法

package com.vincent;import java.util.concurrent.TimeUnit;/** * Vincent 创建于 2016/5/11. */public class SynchronizedDemo {    private Object lock1 = new Object();    private Object lock2 = new Object();    long currentTimeMillis = System.currentTimeMillis();    public synchronized void a() {        long num = System.currentTimeMillis() - currentTimeMillis;        System.out.println("a.num=" + num);        try {            TimeUnit.SECONDS.sleep(2);        } catch (InterruptedException e) {            e.printStackTrace();        }    }    public void b() {        long num = System.currentTimeMillis() - currentTimeMillis;        System.out.println("b.num=" + num);        try {            TimeUnit.SECONDS.sleep(1);        } catch (InterruptedException e) {            e.printStackTrace();        }    }}

输出：

b.num=15b.num=16a.num=16b.num=16b.num=16b.num=16a.num=2016a.num=4016

由此看出：如果有线程访问synchronized方法，其他线程访问非synchronized方法不受影响

• 多线程同时访问多个synchronized关键字修饰的代码块

package com.vincent;import java.util.concurrent.TimeUnit;/** * Vincent 创建于 2016/5/11. */public class SynchronizedDemo {    private Object lock1 = new Object();    private Object lock2 = new Object();    long currentTimeMillis = System.currentTimeMillis();    public void a() {        synchronized (this) {            long num = System.currentTimeMillis() - currentTimeMillis;            System.out.println("a.num=" + num);            try {                TimeUnit.SECONDS.sleep(2);            } catch (InterruptedException e) {                e.printStackTrace();            }        }    }    public void b() {        synchronized (this) {            long num = System.currentTimeMillis() - currentTimeMillis;            System.out.println("b.num=" + num);            try {                TimeUnit.SECONDS.sleep(1);            } catch (InterruptedException e) {                e.printStackTrace();            }        }    }}

输出：

b.num=1a.num=1002a.num=3002a.num=5003b.num=7003b.num=8003b.num=9003b.num=10004

由此可以看出：该用法和示例1作用相同

• synchronized修饰的代码块使用其他同一个对象加锁

package com.vincent;import java.util.concurrent.TimeUnit;/** * Vincent 创建于 2016/5/11. */public class SynchronizedDemo {    private Object lock1 = new Object();    private Object lock2 = new Object();    long currentTimeMillis = System.currentTimeMillis();    public void a() {        synchronized (lock1) {            long num = System.currentTimeMillis() - currentTimeMillis;            System.out.println("a.num=" + num);            try {                TimeUnit.SECONDS.sleep(2);            } catch (InterruptedException e) {                e.printStackTrace();            }        }    }    public void b() {        synchronized (lock1) {            long num = System.currentTimeMillis() - currentTimeMillis;            System.out.println("b.num=" + num);            try {                TimeUnit.SECONDS.sleep(1);            } catch (InterruptedException e) {                e.printStackTrace();            }        }    }}

输出：

b.num=2a.num=1003b.num=3003b.num=4004b.num=5004b.num=6004a.num=7004a.num=9004

结果同1

• synchronized修饰的代码块使用其他不同对象加锁

package com.vincent;import java.util.concurrent.TimeUnit;/** * Vincent 创建于 2016/5/11. */public class SynchronizedDemo {    private Object lock1 = new Object();    private Object lock2 = new Object();    long currentTimeMillis = System.currentTimeMillis();    public void a() {        synchronized (lock1) {            long num = System.currentTimeMillis() - currentTimeMillis;            System.out.println("a.num=" + num);            try {                TimeUnit.SECONDS.sleep(2);            } catch (InterruptedException e) {                e.printStackTrace();            }        }    }    public void b() {        synchronized (lock2) {            long num = System.currentTimeMillis() - currentTimeMillis;            System.out.println("b.num=" + num);            try {                TimeUnit.SECONDS.sleep(1);            } catch (InterruptedException e) {                e.printStackTrace();            }        }    }}

输出：

b.num=1a.num=2b.num=1002b.num=2002a.num=2003b.num=3003b.num=4003a.num=4003

由此看出：访问a和b的线程互不影响