2.1.2实例变量非线程安全

若多个线程共同访问1个对象中的实例变量，那么有可能出现非线程安全的问题。

测试如下

package com.cky.bean;

/**
 * Created by chenkaiyang on 2017/12/4.
 */
public class SelfPrivateNum {
    int num =0;
    public void addI(String usrName) {
        try {
            if (usrName.equals("a")) {
                num  =100;
                System.out.println("a over");
                Thread.sleep(2000);
            } else {
                num= 200;
                System.out.println("b over");

            }
            System.out.println(usrName + " num="+ num);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

package com.cky.thread;

import com.cky.bean.SelfPrivateNum;

/**
 * Created by chenkaiyang on 2017/12/4.
 */
public class ThreadA  extends Thread{
    private SelfPrivateNum sn;
    public ThreadA (SelfPrivateNum sn) {
        this.sn = sn;
    }
    @Override
    public void run() {
        super.run();
        sn.addI("a");
    }
}

package com.cky.thread;

import com.cky.bean.SelfPrivateNum;

/**
 * Created by chenkaiyang on 2017/12/4.
 */
public class ThreadB extends Thread{
    private SelfPrivateNum sn;
    public ThreadB (SelfPrivateNum sn) {
        this.sn = sn;
    }
    @Override
    public void run() {
        super.run();
        sn.addI("b");
    }
}

package com.cky.test;

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

/**
 * Created by chenkaiyang on 2017/12/2.
 */
public class Test {
    public static void main(String[] args){
        SelfPrivateNum sn = new SelfPrivateNum();
        ThreadA threadA = new ThreadA(sn);
        ThreadB threadB = new ThreadB(sn);
        threadA.start();
        threadB.start();
    }
}

D:\it\jdk1.8\bin\java -Didea.launcher.port=7533 "-Didea.launcher.bin.path=D:\it\idea\IntelliJ IDEA 2016.3.3\bin" -Dfile.encoding=UTF-8 -classpath "D:\it\jdk1.8\jre\lib\charsets.jar;D:\it\jdk1.8\jre\lib\deploy.jar;D:\it\jdk1.8\jre\lib\ext\access-bridge-64.jar;D:\it\jdk1.8\jre\lib\ext\cldrdata.jar;D:\it\jdk1.8\jre\lib\ext\dnsns.jar;D:\it\jdk1.8\jre\lib\ext\jaccess.jar;D:\it\jdk1.8\jre\lib\ext\jfxrt.jar;D:\it\jdk1.8\jre\lib\ext\localedata.jar;D:\it\jdk1.8\jre\lib\ext\nashorn.jar;D:\it\jdk1.8\jre\lib\ext\sunec.jar;D:\it\jdk1.8\jre\lib\ext\sunjce_provider.jar;D:\it\jdk1.8\jre\lib\ext\sunmscapi.jar;D:\it\jdk1.8\jre\lib\ext\sunpkcs11.jar;D:\it\jdk1.8\jre\lib\ext\zipfs.jar;D:\it\jdk1.8\jre\lib\javaws.jar;D:\it\jdk1.8\jre\lib\jce.jar;D:\it\jdk1.8\jre\lib\jfr.jar;D:\it\jdk1.8\jre\lib\jfxswt.jar;D:\it\jdk1.8\jre\lib\jsse.jar;D:\it\jdk1.8\jre\lib\management-agent.jar;D:\it\jdk1.8\jre\lib\plugin.jar;D:\it\jdk1.8\jre\lib\resources.jar;D:\it\jdk1.8\jre\lib\rt.jar;F:\springboot\threaddemo\out\production\threaddemo;D:\it\idea\IntelliJ IDEA 2016.3.3\lib\idea_rt.jar" com.intellij.rt.execution.application.AppMain com.cky.test.Test
a over
b over
b num=200
a num=200

Process finished with exit code 0

结果分析：出现了非线程安全的问题，那如何解决呢？

使用synchronized关键字

package com.cky.bean;

/**
 * Created by chenkaiyang on 2017/12/4.
 */
public class SelfPrivateNum {
    int num =0;
   synchronized public void addI(String usrName) {
        try {
            if (usrName.equals("a")) {
                num  =100;
                System.out.println("a over");
                Thread.sleep(2000);
            } else {
                num= 200;
                System.out.println("b over");

            }
            System.out.println(usrName + " num="+ num);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

D:\it\jdk1.8\bin\java -Didea.launcher.port=7534 "-Didea.launcher.bin.path=D:\it\idea\IntelliJ IDEA 2016.3.3\bin" -Dfile.encoding=UTF-8 -classpath "D:\it\jdk1.8\jre\lib\charsets.jar;D:\it\jdk1.8\jre\lib\deploy.jar;D:\it\jdk1.8\jre\lib\ext\access-bridge-64.jar;D:\it\jdk1.8\jre\lib\ext\cldrdata.jar;D:\it\jdk1.8\jre\lib\ext\dnsns.jar;D:\it\jdk1.8\jre\lib\ext\jaccess.jar;D:\it\jdk1.8\jre\lib\ext\jfxrt.jar;D:\it\jdk1.8\jre\lib\ext\localedata.jar;D:\it\jdk1.8\jre\lib\ext\nashorn.jar;D:\it\jdk1.8\jre\lib\ext\sunec.jar;D:\it\jdk1.8\jre\lib\ext\sunjce_provider.jar;D:\it\jdk1.8\jre\lib\ext\sunmscapi.jar;D:\it\jdk1.8\jre\lib\ext\sunpkcs11.jar;D:\it\jdk1.8\jre\lib\ext\zipfs.jar;D:\it\jdk1.8\jre\lib\javaws.jar;D:\it\jdk1.8\jre\lib\jce.jar;D:\it\jdk1.8\jre\lib\jfr.jar;D:\it\jdk1.8\jre\lib\jfxswt.jar;D:\it\jdk1.8\jre\lib\jsse.jar;D:\it\jdk1.8\jre\lib\management-agent.jar;D:\it\jdk1.8\jre\lib\plugin.jar;D:\it\jdk1.8\jre\lib\resources.jar;D:\it\jdk1.8\jre\lib\rt.jar;F:\springboot\threaddemo\out\production\threaddemo;D:\it\idea\IntelliJ IDEA 2016.3.3\lib\idea_rt.jar" com.intellij.rt.execution.application.AppMain com.cky.test.Test
a over
a num=100
b over
b num=200

Process finished with exit code 0

结果分析：使用synchronized关键字，当a拿到锁之后，只有当前执行完毕释放锁的时候，b才能拿到锁，解决了线程安全。而且代码的执行是同步的，因为只有1个对象。