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：
public interface Consumer {
int consume();
}
public class ComsumeThread implements Runnable {
private Consumer consumer;
public ComsumeThread(Consumer consumer) {
this.consumer = consumer;
}
@Override
public void run() {
for(int i=0;i<10;i++){
System.out.println(Thread.currentThread().getName()+" After Consume left:"+consumer.consume());
}
}
}
public class ConsumeClientA implements Consumer {
private static int leftNum = 30;
@Override
public int consume() {
int orgLeftNum = leftNum;
Random random = new Random(System.currentTimeMillis());
try {
Thread.sleep(random.nextInt(3));
} catch (InterruptedException e) {
e.printStackTrace();
}
orgLeftNum = orgLeftNum -1;
leftNum = orgLeftNum;
return leftNum;
}
public static void main(String[] args){
Consumer consumer = new ConsumeClientA();
Thread thread1 = new Thread(new ComsumeThread(consumer));
Thread thread2 = new Thread(new ComsumeThread(consumer));
Thread thread3 = new Thread(new ComsumeThread(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-2 After Consume left:29 Thread-1 After Consume left:29 Thread-3 After Consume left:29 Thread-2 After Consume left:28 Thread-1 After Consume left:28 Thread-3 After Consume left:28 Thread-2 After Consume left:27 Thread-1 After Consume left:27 Thread-2 After Consume left:26 Thread-3 After Consume left:27 Thread-1 After Consume left:25 Thread-2 After Consume left:25 Thread-3 After Consume left:25 Thread-1 After Consume left:24 Thread-2 After Consume left:24 Thread-3 After Consume left:24 Thread-1 After Consume left:23 Thread-2 After Consume left:23 Thread-3 After Consume left:23 Thread-1 After Consume left:22 Thread-2 After Consume left:22 Thread-3 After Consume left:22 Thread-1 After Consume left:21 Thread-2 After Consume left:21 Thread-3 After Consume left:21 Thread-1 After Consume left:20 Thread-2 After Consume left:20 Thread-3 After Consume left:20 Thread-1 After Consume left:19 Thread-3 After Consume left:18
增加threadlocal处理，每个线程相互独立，实现如下：
public class ConsumeClientB implements Consumer {
private ThreadLocal<Integer> leftNumThreadLocal = new ThreadLocal<Integer>(){
@Override
protected Integer initialValue() {
return 30;
}
};
@Override
public int consume() {
int orgLeftNum = leftNumThreadLocal.get();
Random random = new Random(System.currentTimeMillis());
try {
Thread.sleep(random.nextInt(3));
} catch (InterruptedException e) {
e.printStackTrace();
}
orgLeftNum = orgLeftNum -1;
leftNumThreadLocal.set(orgLeftNum);
return leftNumThreadLocal.get();
}
public static void main(String[] args){
Consumer consumer = new ConsumeClientB();
Thread thread1 = new Thread(new ComsumeThread(consumer));
Thread thread2 = new Thread(new ComsumeThread(consumer));
Thread thread3 = new Thread(new ComsumeThread(consumer));
thread1.start();
thread2.start();
thread3.start();
}
}
运行的结果如下：
Thread-1 After Consume left:29 Thread-3 After Consume left:29 Thread-2 After Consume left:29 Thread-1 After Consume left:28 Thread-3 After Consume left:28 Thread-2 After Consume left:28 Thread-1 After Consume left:27 Thread-3 After Consume left:27 Thread-2 After Consume left:27 Thread-1 After Consume left:26 Thread-3 After Consume left:26 Thread-2 After Consume left:26 Thread-1 After Consume left:25 Thread-3 After Consume left:25 Thread-2 After Consume left:25 Thread-1 After Consume left:24 Thread-3 After Consume left:24 Thread-2 After Consume left:24 Thread-1 After Consume left:23 Thread-3 After Consume left:23 Thread-2 After Consume left:23 Thread-1 After Consume left:22 Thread-3 After Consume left:22 Thread-2 After Consume left:22 Thread-1 After Consume left:21 Thread-3 After Consume left:21 Thread-2 After Consume left:21 Thread-1 After Consume left:20 Thread-3 After Consume left:20 Thread-2 After Consume left: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.ThreadLocalMap threadLocals, 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：
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();
}
ThreadLocal.ThreadLocalMap getMap(Thread t) {
return t.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：
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocal.ThreadLocalMap map = getMap(t);
if(map != null) {
map.set(this, value);
} else {
this.createMap(t, value);
}
}
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
static class Entry extends WeakReference<ThreadLocal> {
Object value;
Entry(ThreadLocal var1, Object var2) {
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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