标签：线程安全 并发 thread threadlocal struts2
ThreadLocal是一个和线程安全相关的类。package thread;
public class NotSafeThread implements Runnable{
private int a=10;
public void run() {
// TODO Auto-generated method stub
for (int i = 0; i < 5; i++) {
try {
if (Thread.currentThread().toString()
.equals("Thread[t2,5,main]")
&& i == 2)
Thread.sleep(1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println(Thread.currentThread().toString() + " a="
+ a++);
}
}
}package thread;
public class TestThread {
public static void main(String[] args) {
NotSafeThread nst=new NotSafeThread();
Thread t1=new Thread(nst,"t1");
Thread t2=new Thread(nst,"t2");
Thread t3=new Thread(nst,"t3");
t1.start();
t2.start();
t3.start();
}
}输出的结果:a在各个线程中会累积。public class SafaThread {
private static ThreadLocal<Integer> counterContext=new ThreadLocal<Integer>();
public static Integer getCounterContext(){
if (counterContext.get()!=null)
return counterContext.get();
setCounterContext(10);
return counterContext.get();
}
public static void setCounterContext(Integer value){
counterContext.set(value);
}
public static Integer getCounterContextNext(){
counterContext.set(getCounterContext()+1);
return counterContext.get();
}
}package thread;
public class ThreadLocalTest extends Thread{
public void run() {
for (int i = 0; i < 5; i++)
System.out.println(Thread.currentThread().getName()+" a="+
SafaThread.getCounterContextNext());
}
public static void main(String[] args) {
ThreadLocalTest t=new ThreadLocalTest();
Thread t1=new Thread(t);
Thread t3=new Thread(t);
Thread t2=new Thread(t);
t1.start();
t2.start();
t3.start();
}
}结果如下: static class ThreadLocalMap {
static class Entry extends WeakReference<ThreadLocal> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal k, Object v) {
super(k);
value = v;
}
}
/**
* The table, resized as necessary.
* table.length MUST always be a power of two.
*/
private Entry[] table;
......//省略其他代码
}Entry里面存放的是当前的ThreadLocal和这个线程中的成员变量。
当前线程的成员变量?怎么来的?public class Thread implements Runnable {
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
....//省略代码
}看到了吧,线程这个基类本身里面就有一个ThreadLocalMap。System.out.println(Thread.currentThread().getName()+" a="+
SafaThread.getCounterContextNext());ok,进去 public static Integer getCounterContextNext(){
counterContext.set(getCounterContext()+1);
return counterContext.get();
}最先执行的是getCounterContext()。 public static Integer getCounterContext(){
if (counterContext.get()!=null)
return counterContext.get();
setCounterContext(10);
return counterContext.get();
}我们看看counterContext(它本身是ThreadLocal<Integer>)的get方法。如下: public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t); //标注1
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this); //标注3
if (e != null)
return (T)e.value;
}
return setInitialValue();
}
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}当我们第一次调用SafaThread.getCounterContextNext()的时候,直到上面的标注1处,返回的map肯定为空。我们看看setInitialValue private T setInitialValue() {
T value = initialValue(); //标注2
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
}至于creatMap里面的代码 void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}我们再看看ThreadLocalMap的构造函数。你妹呀,你想累死老子,都最后一步了,自己看! protected T initialValue() {
return null;
}现在我们看看,看看什么?看ThreadLocal的set方法。中间的我不讲了。太累了。 public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}跟get一样是吧。 static class Entry extends WeakReference<ThreadLocal> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal k, Object v) {
super(k);
value = v;
}
}key是ThreadLocal,value是要存储的值。public class SafaThread {
private static ThreadLocal<Integer> counterContext=new ThreadLocal<Integer>();
private static ThreadLocal<Integer> counterContext2=new ThreadLocal<Integer>();
...//省略其他代码 本来有三个方法 现在就是六个
}本来只有一个变量,我现在放两个。
下一个应用
我们看看struts中是如何运用ThreadLocal的。
感谢glt
ThreadLocal最适合使用的场景:
在同一个线程的不同开发层次共享数据
使用ThreadLocal的步骤
1建立一个类,在其中封装静态的ThreadLocal变量,使其成为一个数据共享环境
2在类中实现ThreadLoca变量的静态方法(设值与取值)
先看a再看b
a: http://lavasoft.blog.51cto.com/62575/51926/
b: http://blog.csdn.net/lufeng20/article/details/24314381struts2技术内幕
标签：线程安全 并发 thread threadlocal struts2
原文地址：http://blog.csdn.net/dlf123321/article/details/42531979
