Java中Thread相关的一些小知识
Java使用进程执行shell命令
public static void main(String[] args) throws IOException, InterruptedException {
String[] cmdline = {"sh", "-c", "echo $MYSQL_HOME"};
Runtime runtime = Runtime.getRuntime();
Process process = runtime.exec(cmdline);
BufferedReader read = new BufferedReader(new InputStreamReader(
process.getInputStream()));
try {
process.waitFor();
} catch (InterruptedException e) {
System.out.println(e.getMessage());
}
while (read.ready()) {
System.out.println(read.readLine());
}
}
Runnable的花式写法
public static void main(String[] args) throws InterruptedException {
Thread t1 = new Thread(ThreadCreationDemo::action, "t1");
t1.start();
t1.join();
}
private static void action() {
System.out.println(Thread.currentThread().getName() + " is doing action!");
}
关于Thread的join方法
下面是Thread.join()方法的源码，不难看出主要思路就是自旋(?)+wait。当前调用了thread.join()方法的线程(本地demo一般就是main线程)会进行wait操作，等待notify。
public final synchronized void join(long millis) throws InterruptedException {
long base = System.currentTimeMillis();
long now = 0;
if (millis < 0) {
throw new IllegalArgumentException("timeout value is negative");
}
if (millis == 0) {
while (isAlive()) {
wait(0);
}
} else {
while (isAlive()) {
long delay = millis - now;
if (delay <= 0) {
break;
}
wait(delay);
now = System.currentTimeMillis() - base;
}
}
}
那么，是谁，在什么时候notify了正在wait中的主线程呢？
当前线程在退出之前，会调用Thread.exit()方法,去做一些clean up。
/**
* This method is called by the system to give a Thread
* a chance to clean up before it actually exits.
*/
private void exit() {
if (group != null) {
group.threadTerminated(this);
group = null;
}
/* Aggressively null out all reference fields: see bug 4006245 */
target = null;
/* Speed the release of some of these resources */
threadLocals = null;
inheritableThreadLocals = null;
inheritedAccessControlContext = null;
blocker = null;
uncaughtExceptionHandler = null;
}
他会通知当前线程所在的线程组，清除该线程。线程组清除线程的方式，将该线程从线程组中移除(remove(t))，并且当前线程组没有活跃线程的话，唤醒其他等待的线程。
/**
* Notifies the group that the thread {@code t} has terminated.
*
* <p> Destroy the group if all of the following conditions are
* true: this is a daemon thread group; there are no more alive
* or unstarted threads in the group; there are no subgroups in
* this thread group.
*
* @param t
* the Thread that has terminated
*/
void threadTerminated(Thread t) {
synchronized (this) {
remove(t);
if (nthreads == 0) {
notifyAll();
}
if (daemon && (nthreads == 0) &&
(nUnstartedThreads == 0) && (ngroups == 0))
{
destroy();
}
}
}
所以，Thread.join()方法，会使主线程(本例中)调用wait方法，并在子线程exit后，通过notify()方法唤醒主线程。