我设置了所有线程共有的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，它又重新变为初始值了吗？那是预期的吗？如果是，请解释什么？下面是我的两节课。提前致谢。
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();
}
}
}
输出：
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
最佳答案
ThreadLocal在运行代码的线程（及其关联的线程本地对象）中是本地的。
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
因此，您有三个线程和两个ThreadLocals。您的主线程同时使用本地线程和启动的两个线程，每个线程都有一个值。
每次您有一个新的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办？
在创建另一个ThreadLocal或在另一个线程中使用它之前，您只能读取一次设置的值。
如果前四行在主线程中运行，那么在第2行中设置的内容是否在第3行中没有反映？
第3行是另一个
ThreadLocal中的另一个MyRunnableThreadLocal对象如果它在第一个MyRunnableThreadLocal对象上运行，那么为什么它不在第5或6行上反映呢？
第5和6行是第一次在这些线程中使用那些ThreadLocals，因此它们具有初始值。
注意：如果使用
InheritableThreadLocal，它将按照您的期望进行操作。这是因为新线程“继承”了其父线程设置的值。