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使用synchronized修饰静态方法带来的问题

• 直接上代码

/** * Created with IntelliJ IDEA. * Description: If you don't work hard, you will a loser. * User: Listen-Y. * Date: 2020-09-29 * Time: 11:39 */public class ServiceStatic {
       public synchronized static void printA() {
           System.out.println(System.currentTimeMillis() + "进入printA_Static");        try {
               Thread.sleep(100);        } catch (InterruptedException e) {
               e.printStackTrace();        }        System.out.println(System.currentTimeMillis() + "离开printA_Static");    }    public synchronized static void printB() {
           System.out.println(System.currentTimeMillis() + "进入printB_Static");        try {
               Thread.sleep(100);        } catch (InterruptedException e) {
               e.printStackTrace();        }        System.out.println(System.currentTimeMillis() + "离开printB_Static");    }    public void printC() {
           System.out.println(System.currentTimeMillis() + "进入printC");        try {
               Thread.sleep(100);        } catch (InterruptedException e) {
               e.printStackTrace();        }        System.out.println(System.currentTimeMillis() + "离开printC");    }}

import javax.net.ssl.SSLContext;/** * Created with IntelliJ IDEA. * Description: If you don't work hard, you will a loser. * User: Listen-Y. * Date: 2020-09-29 * Time: 11:41 */public class Run5 {
       public static void main(String[] args) {
           Thread thread = new Thread("A") {
               @Override            public void run() {
                   while (true) {
                       ServiceStatic.printA();                }            }        };        Thread thread1 = new Thread("B") {
               @Override            public void run() {
                   while (true) {
                       ServiceStatic.printB();                }            }        };        ServiceStatic serviceStatic = new ServiceStatic();        Thread thread2 = new Thread("C") {
               @Override            public void run() {
                   while (true) {
                       serviceStatic.printC();                }            }        };        thread.start();        thread1.start();        thread2.start();    }}

• 运行结果

1601380417431进入printA_Static1601380417431进入printC1601380417531离开printA_Static1601380417531离开printC1601380417531进入printC1601380417531进入printA_Static1601380417631离开printC1601380417631进入printC1601380417632离开printA_Static1601380417632进入printA_Static1601380417732离开printC1601380417732进入printC1601380417733离开printA_Static1601380417733进入printB_Static1601380417832离开printC1601380417832进入printC1601380417833离开printB_Static1601380417833进入printB_Static1601380417932离开printC1601380417932进入printC

• 观察我们发现虽然我们给三个方法printA printB printC 都上了锁, 但是运行结果却是printA与printB是同步的, 与printC是异步的
• 这就是给static方法上锁带来的问题, 其实异步的原因是持有不同的锁, 一个是实例对象锁, 一个是类对象锁, 而且类对象锁可以对所有对象实例起到作用, 而printC是只对调用这个方法的那个实例对象其作用

同步synchronized代码块不使用String作为锁对象的原因

• 直接上代码

public class Service {
       public void printStr(String str) {
           //给Str上锁        synchronized (str) {
               while (true) {
                   System.out.println(Thread.currentThread().getName());                try {
                       Thread.sleep(1000);                } catch (InterruptedException e) {
                       e.printStackTrace();                }            }        }    }}

public class ThreadA extends Thread {
       private Service service;    public ThreadA(Service service) {
           this.service = service;    }    @Override    public void run() {
           service.printStr("Listen");    }}class ThreadB extends Thread {
       private Service service;    public ThreadB(Service service) {
           this.service = service;    }    @Override    public void run() {
           service.printStr("Listen");    }}

public class Run4 {
       public static void main(String[] args) {
           Service service = new Service();        //创建俩个线程去执行        ThreadA threadA = new ThreadA(service);        threadA.setName("A");        ThreadB threadB = new ThreadB(service);        threadB.setName("B");        threadA.start();        threadB.start();    }}

• 运行结果

AAAAAAProcess finished with exit code -1

• 分析结果我们发现, A和B俩个线程都启动了, 但是B线程一直没有得到执行, 也就是B线程一直没有获得锁
• 这就是使用String作为锁对象带来的问题, 原因就是JVM中有String常量池缓存的功能, 看下面代码示例

public class Test {
       public static void main(String[] args) {
           String s = "Listen";        String s1 = "Listen";        System.out.println(s==s1);    }}

true

• 由此我们得出, 虽然线程A和线程B传入的不是同一个引用的字符串, 但是这个String对象却是同一个, 也就是上的其实是同一把锁.

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