Topic One: Visibility
public class Demo {
private static Boolean flag = true;
public static void main(String[] args) throws InterruptedException{
new Thread (() -> {
System.out.println ("测试线程启动");
while (flag) {
}
System.out.println ("测试线程结束");
}).start ();
TimeUnit.SECONDS.sleep (1);
flag = false;
Runtime.getRuntime ().addShutdownHook (new Thread (() -> {
System.out.println (Thread.currentThread ().getName () + "钩子线程结束，表示程序退出");
}));
}
}
As follows: main method to start a thread. Inside a while loop, when the main thread is a shared variable is set to false, in order to terminate the thread of the test cycle so that it terminates, so inject a hook thread to determine whether the program terminates, but the visibility is so unkillable , so only one line console print the following information: test thread starts.
So the question is, will change the code a little bit, adding a lock in a while loop, why do shared variables can be met? Question: We all know that really is locked so that the object can be locked so that other threads can be seen, but I have here is just a string lock it, so why have it?
public class Demo {
private static Boolean flag = true;
public static void main(String[] args) throws InterruptedException{
new Thread (() -> {
System.out.println ("测试线程启动");
while (flag) {
synchronized (""){
}
}
System.out.println ("测试线程结束");
}).start ();
TimeUnit.SECONDS.sleep (1);
flag = false;
Runtime.getRuntime ().addShutdownHook (new Thread (() -> {
System.out.println (Thread.currentThread ().getName () + "钩子线程结束，表示程序退出");
}));
}
}测试线程启动
测试线程结束
Thread-1钩子线程结束，表示程序退出
