Question

我为所有线程设置了通用的ThreadLocal初始值。调用构造函数时，我更改了值并正确打印。但是当启动线程时，它又一次变为初始值？这是预期的吗？如果是，那么解释是什么？以下是我的两个班级。提前谢谢。

MyRunnableThreadLocal.java

public class MyRunnableThreadLocal implements Runnable {

private ThreadLocal<String> threadLocal = new ThreadLocal<String>(){
    @Override protected String initialValue() {
        return "Thread Name not set yet";
    }
};
public MyRunnableThreadLocal(String threadName) {
    System.out.println(threadLocal.get());
    threadLocal.set(threadName);
    System.out.println("Thread name: " + threadLocal.get());
}
@Override
public void run() {
    System.out.println(threadLocal.get());
    threadLocal.set(threadLocal.get() + " " + (Math.random() * 100D));
    try {
        Thread.sleep(2000);
    } catch (InterruptedException e) {
    }

    System.out.println(threadLocal.get());
}


}

ThreadLocalTest.java

public class ThreadLocalTest {

public static void main(String[] args) {
    Thread t1 = new Thread(new MyRunnableThreadLocal("Thread 1"));
    Thread t2 = new Thread(new MyRunnableThreadLocal("Thread 2"));

    t1.start();
    t2.start();
    try {
        t1.join();
        t2.join();
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

}

}

Out Put：

Thread Name not set yet
Thread name: Thread 1
Thread Name not set yet
Thread name: Thread 2
Thread Name not set yet
Thread Name not set yet
Thread Name not set yet 20.24825634584746
Thread Name not set yet 59.67633602373702

Answer 1

ThreadLocal是运行代码的线程的本地（以及与之关联的线程本地对象。）

Thread Name not set yet <= runs in main
Thread name: Thread 1   <= runs in main for the first MyRunnableThreadLocal object
Thread Name not set yet <= runs in main for the 2nd MyRunnableThreadLocal object
Thread name: Thread 2   <= runs in main for the 2nd MyRunnableThreadLocal object
Thread Name not set yet <= runs in the first thread for the first MyRunnableThreadLocal
Thread Name not set yet <= runs in the 2nd thread for the 2nd MyRunnableThreadLocal
Thread Name not set yet 20.24825634584746  <= runs in the first thread for the first MyRunnableThreadLocal
Thread Name not set yet 59.67633602373702  <= runs in the 2nd thread for the 2nd MyRunnableThreadLocal

所以你有三个线程和两个ThreadLocals。你的主要使用本地线程和你开始的两个线程各有一个值。

每次有新的ThreadLocal对象或新的Thread时，它都有一个新的初始值。

设置线程本地值怎么样？

在创建另一个ThreadLocal或在另一个线程中使用它之前，您只读过一次设置的值。

如果在主线程中运行前四行，那么＃2行中的设置是否反映在第3行？

第3行是另一个ThreadLocal

中的MyRunnableThreadLocal对象

如果它正在处理第一个MyRunnableThreadLocal对象，那么为什么它没有反映在第5行或第6行？

第5行和第6行是这些ThreadLocals第一次在这些线程中使用，因此它们具有初始值。

注意：如果您使用InheritableThreadLocal，它会按预期执行。这是因为新线程＆＃34;继承＆＃34;由它的父线程设置的值。

ThreadLocal InitialValue被调用两次

1 个答案: