java 中ThreadLocal实例分析

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：

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;

IAzNf 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

ThreIAzNfad-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 leftNumThreadLocal = new ThreadLocal(){

@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 {

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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