2.2.10数据类型String的常量池特性

在JVM中具有String常量池缓存的功能

package com.cky.test;

/**
 * Created by edison on 2017/12/8.
 */
public class Test {
    public static void main(String[] args) {
       String a= "a";
       String b= "a";
        System.out.println(a==b);

    }
}

true

将synchronized(string)同步代码块和String联合使用，结果出现意外

测试

package com.cky.bean;

/**
 * Created by edison on 2017/12/8.
 */
public class Service {

    public static  void print(String str) {
        try {
            synchronized (str) {
                while(true) {
                    System.out.println(Thread.currentThread().getName());
                    Thread.sleep(1000);
                }
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }



}

package com.cky.thread;

import com.cky.bean.Service;

/**
 * Created by edison on 2017/12/8.
 */
public class ThreadA extends  Thread{
    private Service service;

    public ThreadA(Service service) {
        this.service = service;
    }

    @Override
    public void run() {
        super.run();
        service.print("AA");
    }
}

package com.cky.thread;

import com.cky.bean.Service;

/**
 * Created by edison on 2017/12/8.
 */
public class ThreadB extends  Thread{


    private Service service;

    public ThreadB(Service service) {
        this.service = service;
    }

    @Override
    public void run() {
        super.run();
        service.print("AA");
    }
}

package com.cky.test;

import com.cky.bean.Service;
import com.cky.thread.ThreadA;
import com.cky.thread.ThreadB;

/**
 * Created by edison on 2017/12/8.
 */
public class Test {
    public static void main(String[] args) {
        Service service = new Service();
        ThreadA threadA = new ThreadA(service);
        threadA.setName("a");
        threadA.start();
        ThreadB threadB = new ThreadB(service);
        threadB.setName("b");
        threadB.start();

    }
}

a
a
a
a
a
a
a
a

结果是无限循环的打印a

出现这种情况的原因是因为String的两个值都是AA,两个线程持有相同的锁。所以造成线程B不能执行，这就是String常量2池带来的问题，所以大多数情况下都不使用String做所对象，比如new Object实例化一个Object对象，但他不放入缓存中

测试

package com.cky.bean;

/**
 * Created by edison on 2017/12/8.
 */
public class Service {

    public static  void print(Object object) {
        try {
            synchronized (object) {
                while(true) {
                    System.out.println(Thread.currentThread().getName());
                    Thread.sleep(1000);
                }
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }



}

package com.cky.thread;

import com.cky.bean.Service;

/**
 * Created by edison on 2017/12/8.
 */
public class ThreadA extends  Thread{
    private Service service;

    public ThreadA(Service service) {
        this.service = service;
    }

    @Override
    public void run() {
        super.run();
        service.print(new Object());
    }
}

package com.cky.thread;

import com.cky.bean.Service;

/**
 * Created by edison on 2017/12/8.
 */
public class ThreadB extends  Thread{


    private Service service;

    public ThreadB(Service service) {
        this.service = service;
    }

    @Override
    public void run() {
        super.run();
        service.print(new Object());
    }
}

package com.cky.test;

import com.cky.bean.Service;
import com.cky.thread.ThreadA;
import com.cky.thread.ThreadB;

/**
 * Created by edison on 2017/12/8.
 */
public class Test {
    public static void main(String[] args) {
        Service service = new Service();
        ThreadA threadA = new ThreadA(service);
        threadA.setName("a");
        threadA.start();
        ThreadB threadB = new ThreadB(service);
        threadB.setName("b");
        threadB.start();

    }
}

a
b
b
a
a
b
a
b
b
a

交替打印说明一个一个对象锁