2.1.2实例变量非线程安全
如果多个线程共同访问1个对象中的实例变量，则有可能出现“非线程安全”问题。
/**
* @author wuyoushan
* @date 2017/4/4.
*/
public class HasSelfPrivateNum {
private int num=0;
public void addI(String username){
try {
if (username.equals("a")) {
num = 100;
System.out.println("a set over!");
Thread.sleep(200);
}else{
num=200;
System.out.println("b set over!");
}
System.out.println(username+" num="+num);
}catch (Exception ex){
ex.printStackTrace();
}
}
}
/**
* @author wuyoushan
* @date 2017/4/4.
*/
public class ThreadA extends Thread{
private HasSelfPrivateNum numRef;
public ThreadA(HasSelfPrivateNum numRef){
super();
this.numRef=numRef;
}
@Override
public void run() {
super.run();
numRef.addI("a");
}
}
/**
* @author wuyoushan
* @date 2017/4/4.
*/
public class ThreadB extends Thread{
private HasSelfPrivateNum numRef;
public ThreadB(HasSelfPrivateNum numRef){
super();
this.numRef=numRef;
}
@Override
public void run() {
super.run();
numRef.addI("b");
}
}
/**
* @author wuyoushan
* @date 2017/3/20.
*/
public class Run {
public static void main(String[] args){
HasSelfPrivateNum numRef=new HasSelfPrivateNum();
ThreadA threadA=new ThreadA(numRef);
threadA.start();
ThreadB threadB=new ThreadB(numRef);
threadB.start();
}
}
程序运行结果为:
a set over!
b set over!
b num=200
a num=200
本实验是两个线程同时访问一个没有同步的方法，如果两个线程同时操作业务对象中的实例变量，则有可能会出现“非线程安全”问题。修改HasSelfPrivateNum.java文件如下
/**
* @author wuyoushan
* @date 2017/4/4.
*/
public class HasSelfPrivateNum {
private int num=0;
synchronized public void addI(String username){
try {
if (username.equals("a")) {
num = 100;
System.out.println("a set over!");
Thread.sleep(200);
}else{
num=200;
System.out.println("b set over!");
}
System.out.println(username+" num="+num);
}catch (Exception ex){
ex.printStackTrace();
}
}
}
重新运行程序，运行结果为:
a set over!
a num=100
b set over!
b num=200
摘选自 java多线程核心编程技术-2.1.2
