笔者今天趁着项目空隙,学习了下ThreadLocal这个类,探寻了下多线程下如何实现线程安全的原理。代码分享下:
一:未使用ThreadLocal的Demo
package com.suning.sample.transcation;
public class ThreadTest {
/**
* 功能描述: <br>
* 〈功能详细描述〉
*
* @param args
* @see [相关类/方法](可选)
* @since [产品/模块版本](可选)
*
*/
private static int seqNum = 0;
public int getNextNum(){
seqNum ++;
return seqNum;
}
public static void main(String[] args) {
ThreadTest sn = new ThreadTest();
TestClient t1 = new TestClient(sn);
TestClient t2 = new TestClient(sn);
TestClient t3 = new TestClient(sn);
t3.start();
t2.start();
t1.start();
}
private static class TestClient extends Thread{
private ThreadTest sn;
public TestClient(ThreadTest sn){
this.sn = sn;
}
public void run(){
for(int i = 0;i<3;i++){
System.out.println("thread[" + Thread.currentThread().getName() +
"]sn[" +sn.getNextNum() + "]");
}
}
}
}
输出结果:thread[Thread-2]sn[1]
thread[Thread-2]sn[3]
thread[Thread-2]sn[4]
thread[Thread-1]sn[2]
thread[Thread-1]sn[5]
thread[Thread-1]sn[6]
thread[Thread-0]sn[7]
thread[Thread-0]sn[8]
thread[Thread-0]sn[9]
上述线程t1,t2,t3争抢同一个静态变量对象seqNum,并且在run()方法中会连续三次获取seqNum值,每次获取后会对seqNum值+1。Thread-2在获取到seqNum=1后,seqNum+1成为2,立马存在Thread-1访问了seqNum值,并+1使得seqNum=3,以至于Thread-2再次访问seqNum的时候获取的seqNum=3.以上模拟了多线程下资源争抢的情景。二:使用ThreadLocal的Demo
package com.suning.sample.transcation;
public class SequenceNum {
/**
* 功能描述: <br>
* 〈功能详细描述〉
*
* @param args
* @see [相关类/方法](可选)
* @since [产品/模块版本](可选)
*/
// TODO Auto-generated method stub
ThreadLocal<Integer> seqNum = new ThreadLocal<Integer>(){
public Integer initialValue(){
return 0;
}
};
public int getNextNum(){
seqNum.set(seqNum.get()+1);
return seqNum.get();
}
public static void main(String[] args){
SequenceNum sn = new SequenceNum();
TestClient t1 = new TestClient(sn);
TestClient t2 = new TestClient(sn);
TestClient t3 = new TestClient(sn);
t3.start();
t2.start();
t1.start();
}
private static class TestClient extends Thread{
private SequenceNum sn;
public TestClient(SequenceNum sn){
this.sn = sn;
}
public void run(){
for(int i = 0;i<3;i++){
System.out.println("thread[" + Thread.currentThread().getName() +
"]sn[" +sn.getNextNum() + "]");
}
}
}
}
结果:
thread[Thread-1]sn[1]
thread[Thread-1]sn[2]
thread[Thread-1]sn[3]
thread[Thread-2]sn[1]
thread[Thread-2]sn[2]
thread[Thread-2]sn[3]
thread[Thread-0]sn[1]
thread[Thread-0]sn[2]
thread[Thread-0]sn[3]
上述线程t1,t2,t3共享sn,使用同一个seqNum对象,但是线程却没有互相影响,均是各自打印自己的1,2,3.为什么呢?因为ThreadLocal,多线程下会复制一份自己的seqNum,各自线程之间的数据是互不影响的。所以把不安全的变量放以泛型的方式放入ThreadLocal可以解决线程不安全的问题。