早在JDK 1.2的版本中就提供java.lang.ThreadLocal，ThreadLocal为解决多线程程序的并发问题提供了一种新的思路。使用这个工具类可以很简洁地编写出优美的多线程程序。
当使用ThreadLocal维护变量时，ThreadLocal为每个使用该变量的线程提供独立的变量副本，所以每一个线程都可以独立地改变自己的副本，而不会影响其它线程所对应的副本.
最常见的ThreadLocal使用场景为 用来解决 数据库连接、Session管理等.
2.部分源码
ThreadLocal中存在ThreadLocalMap 以不同的线程key创建不同的副本。如下ThreadLocal set() /get()方法
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null)
return (T)e.value;
}
return setInitialValue();
}
3.代码实例
/**
* Created by wuhao on 15-12-23.
*/
public class ThreadLocalDemo {
public static void main(String[] args) {
ThreadStringSeq tss = new ThreadStringSeq();
tss.setState("first");
//切换state
ThreadStringClient ts1 = new ThreadStringClient(tss);
ThreadStringClient ts2 = new ThreadStringClient(tss);
new Thread(ts1).start();
new Thread(ts2).start();
ThreadLocalSeq tls = new ThreadLocalSeq();
System.out.println("Main:" + Thread.currentThread().getName() + " sn:" + tls.getNextState());
//切换state
ThreadLocalClient tl1 = new ThreadLocalClient(tls);
ThreadLocalClient tl2 = new ThreadLocalClient(tls);
new Thread(tl1).start();
new Thread(tl2).start();
}
}
class ThreadLocalSeq {
//通过匿名内部类覆盖ThreadLocal的initialValue()方法，指定初始值
private static ThreadLocal<String> state = new ThreadLocal<String>() {
public String initialValue() {
return "";
}
};
//获取下一个序列值
public String getNextState() {
state.set(state.get() + "-->|");
return state.get();
}
public void setState(String st){
state.set(st);
}
}
class ThreadStringSeq {
//通过匿名内部类覆盖ThreadLocal的initialValue()方法，指定初始值
private static String state = "";
//获取下一个序列值
public String getNextState() {
state = state + "-->|";
return state;
}
public void setState(String st){
state = st;
}
}
class ThreadLocalClient implements Runnable {
private ThreadLocalSeq sn;
public ThreadLocalClient(ThreadLocalSeq sn) {
super();
this.sn = sn;
}
public void run() {
for (int i = 0; i < 3; i++) {
//获取线程状态
System.out.println("ThreadLocalClient:" + Thread.currentThread().getName() + " sn:" + sn.getNextState());
}
}
}
class ThreadStringClient implements Runnable {
private ThreadStringSeq sn;
public ThreadStringClient(ThreadStringSeq sn) {
super();
this.sn = sn;
}
public void run() {
for (int i = 0; i < 3; i++) {
//获取线程状态
System.out.println("ThreadStringClient:" + Thread.currentThread().getName() + " sn:" + sn.getNextState());
}
}
}