目前网上介绍ThreadLocal类用法的博文很多，看过后我们基本可以得出以下结论
ThreadLocal的作用和目的：
用于实现线程内的数据共享，即对于相同的程序代码，多个模块在同一个线程中运行时要共享一份数据，而在另外线程中运行时又共享另外一份数据。
好了，至此ThreadLocal的概念我们弄清了，它是每个线程的菊部变量（该死的输入法）。但是java有了类的局部变量，这个ThreadLocal是否显得多余，我们为什么或者在什么情况下用这个东西？
先看以下两段代码
代码1.使用ThreadLocal
public class ThreadLocalDemo implements Runnable {
private final static ThreadLocal<Integer> tLocal = new ThreadLocal<Integer>();
public static void main(String[] agrs) {
ThreadLocalDemo td = new ThreadLocalDemo();
Thread t1 = new Thread(td, "t1");
Thread t2 = new Thread(td, "t2");
t1.start();
t2.start();
}
/**
* 示例业务方法
*/
public void test() {
while (true) {
try {
Integer b = (Integer) tLocal.get();
if (b == null) {
b = 0;
tLocal.set(b);
}
String currentThreadName = Thread.currentThread().getName();
System.out.println("thread " + currentThreadName + " set value to:" + b++);
tLocal.set(b);
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
@Override
public void run() {
test();
}
}
运行结果：
thread t2 set value to:0 thread t1 set value to:0 thread t1 set value to:1 thread t2 set value to:1 thread t1 set value to:2 thread t2 set value to:2 thread t1 set value to:3 thread t2 set value to:3
代码2.使用局部变量
public class ThreadLocalDemoMutli implements Runnable {
private int a = 0;
public static void main(String[] agrs) {
ThreadLocalDemoMutli td = new ThreadLocalDemoMutli();
ThreadLocalDemoMutli td2 = new ThreadLocalDemoMutli();
Thread t1 = new Thread(td, "t1");
Thread t2 = new Thread(td2, "t2");
t1.start();
t2.start();
}
/**
* 示例业务方法，用来测
*/
public void test() {
while (true) {
try {
String currentThreadName = Thread.currentThread().getName();
System.out.println("thread " + currentThreadName + " set age to:" + a++);
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
@Override
public void run() {
test();
}
}
运行结果
thread t1 set age to:0 thread t2 set age to:0 thread t1 set age to:1 thread t2 set age to:1 thread t2 set age to:2 thread t1 set age to:2 thread t1 set age to:3 thread t2 set age to:3
感觉用不用结果都一样嘛？再看下面代码
代码3
public class ThreadLocalDemoSingle implements Runnable {
private int a = 0;
public static void main(String[] agrs) {
ThreadLocalDemoSingle td = new ThreadLocalDemoSingle();
Thread t1 = new Thread(td, "t1");
Thread t2 = new Thread(td, "t2");
t1.start();
t2.start();
}
/**
* 示例业务方法，用来测
*/
public void test() {
while (true) {
try {
String currentThreadName = Thread.currentThread().getName();
System.out.println("thread " + currentThreadName + " set age to:" + a++);
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
@Override
public void run() {
test();
}
}
运行结果：
thread t1 set age to:0 thread t2 set age to:1 thread t1 set age to:2 thread t2 set age to:3 thread t1 set age to:4 thread t2 set age to:5 thread t1 set age to:6 thread t2 set age to:7 thread t1 set age to:8 thread t2 set age to:9 thread t2 set age to:10 thread t1 set age to:11
这回不一样了，对比代码2与代码3的区别就在于创建线程的时候
代码2里
ThreadLocalDemoMutli td = new ThreadLocalDemoMutli(); ThreadLocalDemoMutli td2 = new ThreadLocalDemoMutli(); Thread t1 = new Thread(td, "t1"); Thread t2 = new Thread(td2, "t2");
代码3里
ThreadLocalDemoSingle td = new ThreadLocalDemoSingle(); Thread t1 = new Thread(td, "t1"); Thread t2 = new Thread(td, "t2");
只不过多new了一个对象，原因也一眼看的清楚，但是始终没回答为什么要用ThreadLocal
看来只能从框架里找找原因了，下面搬出hibernate源码（我已经修改了很多）
代码4
public class HibernateUtil {
//创建线程局部变量session，用来保存Hibernate的Session
public static final ThreadLocal<Integer> session = new ThreadLocal<Integer>();
public static Integer s2=0;
/**
* 获取当前线程中的Session
* @return Session
* @throws HibernateException
*/
public static Integer currentSession() {
Integer s = session.get();
if (s == null) {
s = 0;
session.set(s);
}
s++;
session.set(s);
return s;
}
/**
* 获取当前线程中的Session
* @return Session
* @throws HibernateException
*/
public static Integer currentSessionError() {
return s2++;
}
}
代码5
public class THibernate implements Runnable {
@Override
public void run() {
while (true) {
try {
String currentThreadName = Thread.currentThread().getName();
System.out.println("thread " + currentThreadName +"|tlocal:"+HibernateUtil.currentSession()+"|common:"+HibernateUtil.currentSessionError());
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args) {
THibernate t=new THibernate();
Thread t1 = new Thread(t, "t1");
Thread t2 = new Thread(t, "t2");
t1.start();
t2.start();
}
}
运行结果：
thread t1|tlocal:1|common:0 thread t2|tlocal:1|common:1 thread t1|tlocal:2|common:2 thread t2|tlocal:2|common:3 thread t1|tlocal:3|common:4 thread t2|tlocal:3|common:5 thread t1|tlocal:4|common:6 thread t2|tlocal:4|common:7 thread t1|tlocal:5|common:8 thread t2|tlocal:5|common:9
HibernateUtil是一个可供多线程访问的工具类，从运行结果来看ThreadLocal做到了变量的线程隔离，而普通的类变量无法做到这点。
那我把代码4里的
public static Integer s2=0;
改成
public Integer s2=0;
不就也完成了ThreadLocal做的事情吗
但是，你的方法是static的，用到的变量也需要是static的，在这种情况下，还真的必须用ThreadLocal，才能做到变量的线程隔离.
原创文章，转载请声名出处 http://spjich.iteye.com/blog/2264457