java 中ThreadLocal实例分析
java 中ThreadLocal实例分析
从概念上理解，threadlocal使变量在多个线程中相互隔离实现线程安全，threadlocal包装的变量最终都专属于对应的每个线程，线程之间相互独立，用一个具体实现来说明：
publicinterfaceConsumer{
intconsume();
}
publicclassComsumeThreadimplementsRunnable{
privateConsumerconsumer;
publicComsumeThread(Consumerconsumer){
this.consumer=consumer;
}
@Override
publicvoidrun(){
for(inti=0;i<10;i++){
System.out.println(Thread.currentThread().getName()+"AfterConsumeleft:"+consumer.consume());
}
}
}
publicclassConsumeClientAimplementsConsumer{
privatestaticintleftNum=30;
@Override
publicintconsume(){
intorgLeftNum=leftNum;
Randomrandom=newRandom(System.currentTimeMillis());
try{
Thread.sleep(random.nextInt(3));
}catch(InterruptedExceptione){
e.printStackTrace();
}
orgLeftNum=orgLeftNum-1;
leftNum=orgLeftNum;
returnleftNum;
}
publicstaticvoidmain(String[]args){
Consumerconsumer=newConsumeClientA();
Threadthread1=newThread(newComsumeThread(consumer));
Threadthread2=newThread(newComsumeThread(consumer));
Threadthread3=newThread(newComsumeThread(consumer));
thread1.start();
thread2.start();
thread3.start();
}
}
ConsumeClientA是在没有做任何线程安全处理，结果如下：
Thread-2AfterConsumeleft:29 Thread-1AfterConsumeleft:29 Thread-3AfterConsumeleft:29 Thread-2AfterConsumeleft:28 Thread-1AfterConsumeleft:28 Thread-3AfterConsumeleft:28 Thread-2AfterConsumeleft:27 Thread-1AfterConsumeleft:27 Thread-2AfterConsumeleft:26 Thread-3AfterConsumeleft:27 Thread-1AfterConsumeleft:25 Thread-2AfterConsumeleft:25 Thread-3AfterConsumeleft:25 Thread-1AfterConsumeleft:24 Thread-2AfterConsumeleft:24 Thread-3AfterConsumeleft:24 Thread-1AfterConsumeleft:23 Thread-2AfterConsumeleft:23 Thread-3AfterConsumeleft:23 Thread-1AfterConsumeleft:22 Thread-2AfterConsumeleft:22 Thread-3AfterConsumeleft:22 Thread-1AfterConsumeleft:21 Thread-2AfterConsumeleft:21 Thread-3AfterConsumeleft:21 Thread-1AfterConsumeleft:20 Thread-2AfterConsumeleft:20 Thread-3AfterConsumeleft:20 Thread-1AfterConsumeleft:19 Thread-3AfterConsumeleft:18
增加threadlocal处理，每个线程相互独立，实现如下：
publicclassConsumeClientBimplementsConsumer{
privateThreadLocalleftNumThreadLocal=newThreadLocal(){
@Override
protectedIntegerinitialValue(){
return30;
}
};
@Override
publicintconsume(){
intorgLeftNum=leftNumThreadLocal.get();
Randomrandom=newRandom(System.currentTimeMillis());
try{
Thread.sleep(random.nextInt(3));
}catch(InterruptedExceptione){
e.printStackTrace();
}
orgLeftNum=orgLeftNum-1;
leftNumThreadLocal.set(orgLeftNum);
returnleftNumThreadLocal.get();
}
publicstaticvoidmain(String[]args){
Consumerconsumer=newConsumeClientB();
Threadthread1=newThread(newComsumeThread(consumer));
Threadthread2=newThread(newComsumeThread(consumer));
Threadthread3=newThread(newComsumeThread(consumer));
thread1.start();
thread2.start();
thread3.start();
}
}
运行的结果如下：
Thread-1AfterConsumeleft:29 Thread-3AfterConsumeleft:29 Thread-2AfterConsumeleft:29 Thread-1AfterConsumeleft:28 Thread-3AfterConsumeleft:28 Thread-2AfterConsumeleft:28 Thread-1AfterConsumeleft:27 Thread-3AfterConsumeleft:27 Thread-2AfterConsumeleft:27 Thread-1AfterConsumeleft:26 Thread-3AfterConsumeleft:26 Thread-2AfterConsumeleft:26 Thread-1AfterConsumeleft:25 Thread-3AfterConsumeleft:25 Thread-2AfterConsumeleft:25 Thread-1AfterConsumeleft:24 Thread-3AfterConsumeleft:24 Thread-2AfterConsumeleft:24 Thread-1AfterConsumeleft:23 Thread-3AfterConsumeleft:23 Thread-2AfterConsumeleft:23 Thread-1AfterConsumeleft:22 Thread-3AfterConsumeleft:22 Thread-2AfterConsumeleft:22 Thread-1AfterConsumeleft:21 Thread-3AfterConsumeleft:21 Thread-2AfterConsumeleft:21 Thread-1AfterConsumeleft:20 Thread-3AfterConsumeleft:20 Thread-2AfterConsumeleft:20
每个线程拥有自己的独立变量，相互隔离实现线程安全。
那ThreadLocal是怎样实现这种线程隔离的线程安全的呢？
从ThreadLocal源码可以看到，真正实现线程隔离，与线程挂钩的，其实是ThreadLocal.ThreadLocalMap这个实现类，最明显的体现就在于Thread类源码的这样一个变量申明说明了ThreadLocal.ThreadLocalMap与Thread的关系：
ThreadLocal.ThreadLocalMapthreadLocals,inheritableThreadLocals;
Thread类是包含threadLocals对象的，ThreadLocal的具体实现就是根据提供的get,set等接口，对当前thread的threadLocals变量进行相关操作的，如get操作代码如下：
publicTget(){
Threadt=Thread.currentThread();
ThreadLocalMapmap=getMap(t);
if(map!=null){
ThreadLocalMap.Entrye=map.getEntry(this);
if(e!=null)
return(T)e.value;
}
returnsetInitialValue();
}
ThreadLocal.ThreadLocalMapgetMap(Threadt){
returnt.threadLocals;
}
可以看到，getMap()方法就是从当前thread获取对应的threadLocals变量，然后从这个ThreadLocal.ThreadLocalMap类型的threadLocals变量中获取对应线程中该ThreadLocal对象对应的变量值。
set方法的操作也是一样：
publicvoidset(Tvalue){
Threadt=Thread.currentThread();
ThreadLocal.ThreadLocalMapmap=getMap(t);
if(map!=null){
map.set(this,value);
}else{
this.createMap(t,value);
}
}
voidcreateMap(Threadt,TfirstValue){
t.threadLocals=newThreadLocalMap(this,firstValue);
}
staticclassEntryextendsWeakReference{ Objectvalue; Entry(ThreadLocalvar1,Objectvar2){ super(var1); this.value=var2; } }
ThreadLocalMap中存的是内部类Entry的数组,Entry是继承WeakReference实现，WeakReference的好处是保存对象引用，而又不干扰该对象被GC回收，线程执行完回收threadLocals变量时不会受到Entry封装的变量的干扰。
而且ThreadLocalMap中的key是ThreadLocal，所以一个ThreadLocal对象只能在一个Thread对象中保存一个ThreadLocal的value。
综上，很多人说ThreadLocal的实现是ThreadLocalMap中存Thread对象为key，变量为value的map结构，其实是错误的。
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