這樣終止執行緒，竟然會導致服務宕機？
* If there is a security manager installed, its checkAccess
this
* as its argument. This may result in a
* SecurityException being raised (in the current thread).
*
* If this thread is different from the current thread (that is, the current
* thread is trying to stop a thread other than itself), the
* security manager's checkPermission
RuntimePermission("stopThread") argument) is called in
* addition.
* Again, this may result in throwing a
* SecurityException (in the current thread).
*
* The thread represented by this thread is forced to stop whatever
* it is doing abnormally and to throw a newly created
* ThreadDeath object as an exception.
*
* It is permitted to stop a thread that has not yet been started. * If the thread is eventually started, it immediately terminates. *
* An application should not normally try to catch
* ThreadDeath unless it must do some extraordinary
* cleanup operation (note that the throwing of
* ThreadDeath causes finally clauses of
* try statements to be executed before the thread
* officially dies). If a catch clause catches a
* ThreadDeath object, it is important to rethrow the
* object so that the thread actually dies.
*
* The top-level error handler that reacts to otherwise uncaught
* exceptions does not print out a message or otherwise notify the
* application if the uncaught exception is an instance of
* ThreadDeath.
*
* @exception SecurityException if the current thread cannot
* modify this thread.
* @see #interrupt()
* @see #checkAccess()
* @see #run()
* @see #start()
* @see ThreadDeath
* @see ThreadGroup#uncaughtException(Thread,Throwable)
* @see SecurityManager#checkAccess(Thread)
* @see SecurityManager#checkPermission
* @deprecated This method is inherently unsafe. Stopping a thread with
* Thread.stop causes it to unlock all of the monitors that it
* has locked (as a natural consequence of the unchecked
* ThreadDeath exception propagating up the stack). If
* any of the objects previously protected by these monitors were in
* an inconsistent state, the damaged objects become visible to
* other threads, potentially resulting in arbitrary behavior. Many
* uses of stop should be replaced by code that simply
* modifies some variable to indicate that the target thread should
* stop running. The target thread should check this variable
* regularly, and return from its run method in an orderly fashion
* if the variable indicates that it is to stop running. If the
* target thread waits for long periods (on a condition variable,
* for example), the interrupt method should be used to
* interrupt the wait.
* For more information, see
* Why
* are Thread.stop, Thread.suspend and Thread.resume Deprecated?.
*/
@Deprecated
public final void stop() {
SecurityManager security = System.getSecurityManager();
if (security != null) {
checkAccess();
if (this != Thread.currentThread()) {
security.checkPermission(SecurityConstants.STOP_THREAD_PERMISSION);
}
}
// A zero status value corresponds to "NEW", it can't change to
// not-NEW because we hold the lock.
if (threadStatus != 0) {
resume(); // Wake up thread if it was suspended; no-op otherwise
}
// The VM can handle all thread states
stop0(new ThreadDeath());
}
```
可以看出 `stop()` 方法被 `@Deprecated` 註釋修飾了，而被此註解修飾的程式碼表示為過時方法，不建議被使用。從 `stop()` 的備註資訊可以看出，官方也不建議使用 `stop()` ，說它是一個非安全的方法。
## 正確終止執行緒
那如何終止執行緒呢？這裡提供 2 個正確的方法：
1. 設定退出標識退出執行緒；
1. 使用 `interrupt()` 方法終止執行緒。
### 1.自定義退出標識
我們可以自定義一個布林變數來標識是否需要退出執行緒，實現程式碼如下：
```java
// 自定義退出標識退出執行緒
static class FlagThread extends Thread {
public volatile boolean exit = false;
public void run() {
while (!exit) {
// 執行正常的業務邏輯
}
}
}
```
可以看出我們使用了關鍵字 `volatile` 對執行緒進行了修飾，這樣就可以保證多執行緒的執行安全了，在我們需要讓執行緒退出時，只需要把變數 `exit` 賦值為 `true` 就可以了。
### 2.interrupt 終止執行緒
當我們使用 `interrupt()` 方法時，以上兩個示例的執行結果就正常了，執行程式碼如下：
```java
public class ThreadStopExample {
public static void main(String[] args) throws InterruptedException {
// 問題一：破壞了程式的完整性
Thread t1 = new Thread(() -> {
try {
System.out.println("子執行緒開始執行");
// 模擬業務處理
Thread.sleep(1000);
} catch (Exception e) { }
// 虛擬碼：重要業務方法
System.out.println("子執行緒的重要業務方法");
});
t1.start();
// 讓子執行緒先執行一點業務
Thread.sleep(100);
// 終止子執行緒
t1.interrupt();
// 等待一段時間，確保子執行緒“執行完”
Thread.sleep(3000);
System.out.println("主執行緒執行完成");
// 問題二：破壞了原子邏輯
MyThread myThread = new MyThread();
Thread t2 = new Thread(myThread);
// 開啟執行緒
t2.start();
for (int i = 0; i < 10; i++) {
Thread t = new Thread(myThread);
t.start();
}
// 結束執行緒
t2.interrupt();
}
/**
* 自定義原子測試執行緒
*/
static class MyThread implements Runnable {
// 計數器
int num = 0;
@Override
public void run() {
// 同步程式碼塊，保證原子操作
synchronized (MyThread.class) {
// 自增
num++;
try {
// 執行緒休眠 0.1 秒
Thread.sleep(100);
} catch (InterruptedException e) {
System.out.println(e.getMessage());
}
// 自減
num--;
System.out.println(Thread.currentThread().getName() + " | num=" + num);
}
}
}
}
```
以上程式的執行結果為：
> 子執行緒開始執行
>
> 子執行緒的重要業務方法
>
> 主執行緒執行完成
>
> sleep interrupted
>
> Thread-1 | num=0
>
> Thread-9 | num=0
>
> Thread-10 | num=0
>
> Thread-7 | num=0
>
> Thread-6 | num=0
>
> Thread-5 | num=0
>
> Thread-4 | num=0
>
> Thread-2 | num=0
>
> Thread-3 | num=0
>
> Thread-11 | num=0
>
> Thread-8 | num=0
>
可以看出以上的執行都符合我們的預期，這才是正確的終止執行緒的方式。
## 總結
本文我們講了執行緒的三種終止方式，自定義退出標識的方式、使用 `stop()` 的方式或 `interrupt()` 的方式。其中 `stop()` 的方式會導致程式的完整性和原子性被破壞的問題，並且此方法被 JDK 標識為過期方法，不建議使用，而 `interrupt()` 方法無疑是最適合我們的終止執行緒的