Java ThreadLocal线程变量

ThreadLocal

        ThreadLocal线程变量是一个以ThreadLocal对象为键，任意对象为值的存储结构，这个结构被附带在线程上，一个线程可以根据一个ThreadLocal对象查询到绑定到这个线程上的一个值，该值是线程私有数据，线程之间具有隔离性。

验证隔离性

import java.util.concurrent.TimeUnit;
public class ThreadLocalTest {
	public static ThreadLocal<String> threadLocal = new ThreadLocal<String>();
	public static void main(String args[]) throws Exception {
		Thread threadA = new Thread(new ThreadLocalRunner(), "ThreadA");
		Thread threadB = new Thread(new ThreadLocalRunner(), "ThreadB");
		threadA.start();
		threadB.start();
	}

	static class ThreadLocalRunner implements Runnable {
		@Override
		public void run() {
			for (int i = 0; i < 10; i++) {
				threadLocal.set(Thread.currentThread().getName() + " " + String.valueOf(i));
				System.out.println(Thread.currentThread().getName() + " get value = " + threadLocal.get());
				try {
					TimeUnit.SECONDS.sleep(1);
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
			}
		}
	}
}

运行结果

ThreadA get value = ThreadA 0
ThreadB get value = ThreadB 0
ThreadB get value = ThreadB 1
ThreadA get value = ThreadA 1
ThreadB get value = ThreadB 2
ThreadA get value = ThreadA 2
ThreadA get value = ThreadA 3
ThreadB get value = ThreadB 3
ThreadB get value = ThreadB 4
ThreadA get value = ThreadA 4
ThreadB get value = ThreadB 5
ThreadA get value = ThreadA 5
ThreadB get value = ThreadB 6
ThreadA get value = ThreadA 6
ThreadA get value = ThreadA 7
ThreadB get value = ThreadB 7
ThreadB get value = ThreadB 8
ThreadA get value = ThreadA 8
ThreadA get value = ThreadA 9
ThreadB get value = ThreadB 9

InheritableThreadLocal

使用InheritableThreadLocal可以在子线程中取得父线程继承下来的值。

when a child thread is created, the child receives initial values for all inheritable thread-local variables for which the parent has values

当一个子线程创建的时候，子线程接收所有父线程具有的可以继承的thread-local变量值作为初始值。

Normally the child's values will be identical to the parent's; however, the child's value can be made an arbitrary function of the parent's by overriding the childValue() method in this class.

通常，子线程的值将会跟父线程的值是相同的，但是子线程可以通过覆盖这个类的childValue()方法，将子线程的值修改成任意值。

示例代码

public class ThreadLocalTest2 {
	public static InheritableThreadLocalExt inheritableThreadLocal = new InheritableThreadLocalExt();

	static class InheritableThreadLocalExt extends InheritableThreadLocal<String> {
		@Override
		protected String childValue(String parentValue) {
			return parentValue + " changed by child";
		}
	}

	public static void main(String args[]) throws Exception {
		inheritableThreadLocal.set("Thread Main");
		ThreadLocalRunnerSet threadLocalRunnerSet = new ThreadLocalRunnerSet();
		Thread threadA = new Thread(threadLocalRunnerSet, "ThreadA");
		threadA.start();
		try {
			TimeUnit.SECONDS.sleep(5);
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		System.out.println("Thread Main get value  = " + inheritableThreadLocal.get());
	}
	static class ThreadLocalRunnerSet implements Runnable {
		@Override
		public void run() {
			System.out.println(Thread.currentThread().getName() + " get value from parent thread = "
					+ inheritableThreadLocal.get());
			for (int i = 0; i < 10; i++) {

				inheritableThreadLocal.set(Thread.currentThread().getName() + " " + String.valueOf(i));
				System.out.println(
						Thread.currentThread().getName() + " get value from its own = " + inheritableThreadLocal.get());

			}
		}
	}
}

运行结果

ThreadA get value from parent thread = Thread Main changed by child
ThreadA get value from its own = ThreadA 0
ThreadA get value from its own = ThreadA 1
ThreadA get value from its own = ThreadA 2
ThreadA get value from its own = ThreadA 3
ThreadA get value from its own = ThreadA 4
ThreadA get value from its own = ThreadA 5
ThreadA get value from its own = ThreadA 6
ThreadA get value from its own = ThreadA 7
ThreadA get value from its own = ThreadA 8
ThreadA get value from its own = ThreadA 9
Thread Main get value  = Thread Main

扩展：ThreadLocal实现原理