1 简介

　　synchronized在方法内，同步代码块，传入对象，使用的是对象锁，传入class对象，使用的是类锁

　　作用于普通方法，也是对象锁，当前对象

　　作用于静态方法，是类锁

 

2 同步方法示例

2.1两个synchronized 修饰的普通方法

public class SychTest1 {

</span><span style="color: #0000ff;">static</span> <span style="color: #0000ff;">class</span><span style="color: #000000;"> Phone{
    synchronized </span><span style="color: #0000ff;">void</span><span style="color: #000000;">  sendEmail(){  //相当于synchromized(this)
        System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">EMAIL</span><span style="color: #800000;">"</span><span style="color: #000000;">);
    }
    synchronized </span><span style="color: #0000ff;">void</span><span style="color: #000000;"> sendMsg(){
        System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">MSG</span><span style="color: #800000;">"</span><span style="color: #000000;">);
    }
}

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(String[] args) {

    Phone p </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> Phone();

    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        p.sendEmail();
    }).start();



　　　　　　//等待200

try { Thread.sleep(200); } catch (InterruptedException e) { e.printStackTrace(); }


    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        p.sendMsg();
    }).start();
}



}

执行结果

EMAIL
MSG

 

2.2两个synchronized 修饰的普通方法，sendEmail睡了2秒

public class SychTest2 {

</span><span style="color: #0000ff;">static</span> <span style="color: #0000ff;">class</span><span style="color: #000000;"> Phone{
    synchronized </span><span style="color: #0000ff;">void</span><span style="color: #000000;"> sendEmail(){
        </span><span style="color: #0000ff;">try</span> { Thread.sleep(<span style="color: #800080;">2000</span>); } <span style="color: #0000ff;">catch</span><span style="color: #000000;"> (InterruptedException e) { e.printStackTrace(); }
        System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">EMAIL</span><span style="color: #800000;">"</span><span style="color: #000000;">);
    }
    synchronized </span><span style="color: #0000ff;">void</span><span style="color: #000000;"> sendMsg(){
        System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">MSG</span><span style="color: #800000;">"</span><span style="color: #000000;">);
    }
}

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(String[] args) {

    Phone p </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> Phone();

    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        p.sendEmail();
    }).start();
    
    </span><span style="color: #0000ff;">try</span> { Thread.sleep(<span style="color: #800080;">200</span>); } <span style="color: #0000ff;">catch</span><span style="color: #000000;"> (InterruptedException e) { e.printStackTrace(); }
    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        p.sendMsg();
    }).start();
}



}

执行结果，发现第一个线程调用的sendEmail方法虽然等待了2秒，还是先打印，因为它先获取了锁，第二个线程只能等待。它们竞争的是同一个锁对象p

EMAIL
MSG

 

2.3一个同步方法，一个普通方法

public class SychTest3 {

</span><span style="color: #0000ff;">static</span> <span style="color: #0000ff;">class</span><span style="color: #000000;"> Phone{
    synchronized </span><span style="color: #0000ff;">void</span><span style="color: #000000;"> sendEmail(){
        </span><span style="color: #0000ff;">try</span> { Thread.sleep(<span style="color: #800080;">2000</span>); } <span style="color: #0000ff;">catch</span><span style="color: #000000;"> (InterruptedException e) { e.printStackTrace(); }
        System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">EMAIL</span><span style="color: #800000;">"</span><span style="color: #000000;">);
    }
    </span><span style="color: #0000ff;">void</span><span style="color: #000000;"> sendMsg(){
        System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">MSG</span><span style="color: #800000;">"</span><span style="color: #000000;">);
    }

}

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(String[] args) {

    Phone p </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> Phone();

    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        p.sendEmail();
    }).start();


    </span><span style="color: #0000ff;">try</span> { Thread.sleep(<span style="color: #800080;">200</span>); } <span style="color: #0000ff;">catch</span><span style="color: #000000;"> (InterruptedException e) { e.printStackTrace(); }

    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        p.sendMsg();
    }).start();
}



}

执行结果

MSG
EMAIL

 

2.4两个同步方法，但是是两个不同的对象p和p2去调用同步方法

public class SychTest4 {

</span><span style="color: #0000ff;">static</span> <span style="color: #0000ff;">class</span><span style="color: #000000;"> Phone{
    synchronized </span><span style="color: #0000ff;">void</span><span style="color: #000000;"> sendEmail(){
        </span><span style="color: #0000ff;">try</span> { Thread.sleep(<span style="color: #800080;">2000</span>); } <span style="color: #0000ff;">catch</span><span style="color: #000000;"> (InterruptedException e) { e.printStackTrace(); }
        System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">EMAIL</span><span style="color: #800000;">"</span><span style="color: #000000;">);
    }
    synchronized </span><span style="color: #0000ff;">void</span><span style="color: #000000;"> sendMsg(){
        System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">MSG</span><span style="color: #800000;">"</span><span style="color: #000000;">);
    }
    
}

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(String[] args) {

    Phone p </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> Phone();

    Phone p2 </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> Phone();

    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        p.sendEmail();
    }).start();


    </span><span style="color: #0000ff;">try</span> { Thread.sleep(<span style="color: #800080;">200</span>); } <span style="color: #0000ff;">catch</span><span style="color: #000000;"> (InterruptedException e) { e.printStackTrace(); }

    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        p2.sendMsg();
    }).start();
}



}

执行结果，持有的是不同的锁对象p和p2

MSG
EMAIL

 

2.5两个静态同步方法

public class SychTest5 {

</span><span style="color: #0000ff;">static</span> <span style="color: #0000ff;">class</span><span style="color: #000000;"> Phone{
    synchronized </span><span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> sendEmail(){ //相当于synchronized(Phone.class)
        </span><span style="color: #0000ff;">try</span> { Thread.sleep(<span style="color: #800080;">2000</span>); } <span style="color: #0000ff;">catch</span><span style="color: #000000;"> (InterruptedException e) { e.printStackTrace(); }
        System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">EMAIL</span><span style="color: #800000;">"</span><span style="color: #000000;">);
    }
    synchronized </span><span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> sendMsg(){
        System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">MSG</span><span style="color: #800000;">"</span><span style="color: #000000;">);
    }
}

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(String[] args) {

    Phone p </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> Phone();

    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        p.sendEmail();
    }).start();


    </span><span style="color: #0000ff;">try</span> { Thread.sleep(<span style="color: #800080;">200</span>); } <span style="color: #0000ff;">catch</span><span style="color: #000000;"> (InterruptedException e) { e.printStackTrace(); }

    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        p.sendMsg();
    }).start();
}



}

执行结果，持有的是同一个类锁

EMAIL
MSG

 

2.6 两个静态同步方法，两个对象p和p2去调用

public class SychTest6 {//?

<span style="color: #0000ff;">static</span> <span style="color: #0000ff;">class</span><span style="color: #000000;"> Phone{
    synchronized </span><span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> sendEmail(){
        </span><span style="color: #0000ff;">try</span> { Thread.sleep(<span style="color: #800080;">2000</span>); } <span style="color: #0000ff;">catch</span><span style="color: #000000;"> (InterruptedException e) { e.printStackTrace(); }
        System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">EMAIL</span><span style="color: #800000;">"</span><span style="color: #000000;">);
    }
    synchronized </span><span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> sendMsg(){
        System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">MSG</span><span style="color: #800000;">"</span><span style="color: #000000;">);
    }
}

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(String[] args) {

    Phone p </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> Phone();
    Phone p2 </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> Phone();

    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        p.sendEmail();
    }).start();


    </span><span style="color: #0000ff;">try</span> { Thread.sleep(<span style="color: #800080;">200</span>); } <span style="color: #0000ff;">catch</span><span style="color: #000000;"> (InterruptedException e) { e.printStackTrace(); }

    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        p2.sendMsg();
    }).start();
}



}

执行结果，持有的是同一个类锁，和哪个对象去调用无关

EMAIL
MSG

 

2.7一个静态同步，一个非静态同步

public class SychTest7 {

</span><span style="color: #0000ff;">static</span> <span style="color: #0000ff;">class</span><span style="color: #000000;"> Phone{
    synchronized </span><span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> sendEmail(){
        </span><span style="color: #0000ff;">try</span> { Thread.sleep(<span style="color: #800080;">2000</span>); } <span style="color: #0000ff;">catch</span><span style="color: #000000;"> (InterruptedException e) { e.printStackTrace(); }
        System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">EMAIL</span><span style="color: #800000;">"</span><span style="color: #000000;">);
    }
    synchronized  </span><span style="color: #0000ff;">void</span><span style="color: #000000;"> sendMsg(){
        System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">MSG</span><span style="color: #800000;">"</span><span style="color: #000000;">);
    }
}

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(String[] args) {

    Phone p </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> Phone();

    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        p.sendEmail();
    }).start();


    </span><span style="color: #0000ff;">try</span> { Thread.sleep(<span style="color: #800080;">200</span>); } <span style="color: #0000ff;">catch</span><span style="color: #000000;"> (InterruptedException e) { e.printStackTrace(); }

    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        p.sendMsg();
    }).start();
}



}

执行结果，一个持有类锁，一个持有对象锁

MSG
EMAIL

 

2.8一个静态同步方法，一个非静态同步方法，两个对象去调用

public class SychTest8 {

</span><span style="color: #0000ff;">static</span> <span style="color: #0000ff;">class</span><span style="color: #000000;"> Phone{
    synchronized </span><span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> sendEmail(){
        </span><span style="color: #0000ff;">try</span> { Thread.sleep(<span style="color: #800080;">2000</span>); } <span style="color: #0000ff;">catch</span><span style="color: #000000;"> (InterruptedException e) { e.printStackTrace(); }
        System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">EMAIL</span><span style="color: #800000;">"</span><span style="color: #000000;">);
    }
    synchronized  </span><span style="color: #0000ff;">void</span><span style="color: #000000;"> sendMsg(){
        System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">MSG</span><span style="color: #800000;">"</span><span style="color: #000000;">);
    }
}

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(String[] args) {

    Phone p </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> Phone();

    Phone p2 </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> Phone();


    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        p.sendEmail();
    }).start();


    </span><span style="color: #0000ff;">try</span> { Thread.sleep(<span style="color: #800080;">200</span>); } <span style="color: #0000ff;">catch</span><span style="color: #000000;"> (InterruptedException e) { e.printStackTrace(); }

    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        p2.sendMsg();
    }).start();
}



}

执行结果，一个持有类锁，一个持有对象锁

MSG
EMAIL

 

3 同步代码示例

3.1传入class对象

public class SychTest13 {

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> synchronized  <span style="color: #0000ff;">void</span><span style="color: #000000;"> aa()  {
    System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">aaa</span><span style="color: #800000;">"</span><span style="color: #000000;">);
}

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(String[] args) throws InterruptedException {

    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        synchronized (SychTest13.</span><span style="color: #0000ff;">class</span><span style="color: #000000;">){
            </span><span style="color: #0000ff;">try</span> { Thread.sleep(<span style="color: #800080;">1000</span>); } <span style="color: #0000ff;">catch</span><span style="color: #000000;"> (InterruptedException e) { e.printStackTrace(); }
            System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">aaaaaaaaaaaa</span><span style="color: #800000;">"</span><span style="color: #000000;">);
        }
    }).start();

    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        synchronized (SychTest13.</span><span style="color: #0000ff;">class</span><span style="color: #000000;">){
            System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">bbbbbbbbbb</span><span style="color: #800000;">"</span><span style="color: #000000;">);
        }
    }).start();
}



}

执行结果

aaaaaaaaaaaa
bbbbbbbbbbb

 

3.2传入同一个对象t1

public class SychTest14 {

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> synchronized  <span style="color: #0000ff;">void</span><span style="color: #000000;"> aa()  {
    System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">aaa</span><span style="color: #800000;">"</span><span style="color: #000000;">);
}

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(String[] args) throws InterruptedException {
    SychTest13 t1 </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> SychTest13();
    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        synchronized (t1){
            </span><span style="color: #0000ff;">try</span> { Thread.sleep(<span style="color: #800080;">1000</span>); } <span style="color: #0000ff;">catch</span><span style="color: #000000;"> (InterruptedException e) { e.printStackTrace(); }
            System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">aaaaaaaaaaaa</span><span style="color: #800000;">"</span><span style="color: #000000;">);
        }
    }).start();

    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        synchronized (t1){
            System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">bbbbbbbbbb</span><span style="color: #800000;">"</span><span style="color: #000000;">);
        }
    }).start();
}



}

执行结果

aaaaaaaaaaaa
bbbbbbbbbbb

 

3.3传入对象t1和t2

public class SychTest15 {

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> synchronized  <span style="color: #0000ff;">void</span><span style="color: #000000;"> aa()  {
    System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">aaa</span><span style="color: #800000;">"</span><span style="color: #000000;">);
}

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(String[] args) throws InterruptedException {
    SychTest13 t1 </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> SychTest13();
    SychTest13 t2 </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> SychTest13();
    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        synchronized (t1){
            </span><span style="color: #0000ff;">try</span> { Thread.sleep(<span style="color: #800080;">1000</span>); } <span style="color: #0000ff;">catch</span><span style="color: #000000;"> (InterruptedException e) { e.printStackTrace(); }
            System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">aaaaaaaaaaaa</span><span style="color: #800000;">"</span><span style="color: #000000;">);
        }
    }).start();

    </span><span style="color: #0000ff;">new</span> Thread(()-&gt;<span style="color: #000000;">{
        synchronized (t2){
            System.</span><span style="color: #0000ff;">out</span>.println(<span style="color: #800000;">"</span><span style="color: #800000;">bbbbbbbbbb</span><span style="color: #800000;">"</span><span style="color: #000000;">);
        }
    }).start();
}



}

执行结果

bbbbbbbbbb
aaaaaaaaaaaa

 

4 注意事项

4.1 异常

　　锁会被释放，其它线程可能进来继续执行。
　　会出现数据不一致的情况,多个servlet线程访问同一个资源时,如果第一个线程抛出异常,其他线程就会进入同步代码块,有可能会访问到异常产生时的数据(异常产生的数据很可能是不对的),出现数据不一致的情况

 

4.2 锁对象

　　synchronized (obj)，obj不要用String、常量、基本数据类型。
　　因为使用字符串、常量，你可以使用，其它人也可以使用，你们很可能都不知道对方使用了，那么就有问题了