JAVA线程本地变量ThreadLocal和私有变量的区别

ThreadLocal并不是一个Thread，而是Thread的局部变量，也许把它命名为ThreadLocalVariable更容易让人理解一些。

所以，在Java中编写线程局部变量的代码相对来说要笨拙一些，因此造成线程局部变量没有在Java开发者中得到很好的普及。

ThreadLocal的接口方法

ThreadLocal类接口很简单，只有4个方法，我们先来了解一下：

void set(Object value)

public void remove()

将当前线程局部变量的值删除，目的是为了减少内存的占用。

 

ThreadLocal的原理

在ThreadLocal类中有一个Map，用于存储每一个线程的变量的副本。比如下面的示例实现：

public class ThreadLocal

private Map values = Collections.synchronizedMap(new HashMap());

public Object get()

{

Thread curThread = Thread.currentThread();

Object o = values.get(curThread);

if (o == null && !values.containsKey(curThread))

{

o = initialValue();

values.put(curThread, o);

}

values.put(Thread.currentThread(), newValue);

return o ;

}

public Object initialValue()

{

return null;

}

}

 

 

演示代码

 

public class ThreadLocalExample {

    public static class MyRunnable implements Runnable {

        private ThreadLocal<Integer> threadLocal =

        new ThreadLocal<Integer>();
        int local = 0;

        @Override
        public void run() {

            threadLocal.set((int) (Math.random() * 100D));
            local = (int) (Math.random() * 100D);

            try {

                Thread.sleep(2000);

            } catch (InterruptedException e) {

            }

            System.out.println("threadLocal:" +threadLocal.get());
            System.out.println("local:" +local);

        }

    }

    public static void main(String[] args) {

        MyRunnable sharedRunnableInstance = new MyRunnable();

        Thread thread1 = new Thread(sharedRunnableInstance);

        Thread thread2 = new Thread(sharedRunnableInstance);

        thread1.start();

        thread2.start();

        try {
            thread1.join();
            thread2.join(); 
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } // wait for thread 1 to terminate

        // wait for thread 2 to terminate

    }

}