面試官：如何正確的執行與終止線程池？
01 通常線程池的執行方法有兩種execute和submit，那麼他們有什麼區別呢？
1.接受任務的類型
execute 只能接受Runnable類型的任務
void execute(Runnable command);
submit不管是Runnable還是Callable類型的任務都可以接受，但是Runnable返回值均為void，所以使用Future的get()獲得的還是null
<T> Future<T> submit(Callable<T> task);
<T> Future<T> submit(Runnable task, T result);
Future<?> submit(Runnable task);
2.execute方法適用於那些不需要關注返回值的場景，我們只需要把要執行的任務丟進線程池
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class Thread02 {
public static void main(String[] args) throws Exception {
ExecutorService pool = Executors.newFixedThreadPool(1);
pool.execute(() -> {
System.out.println("我愛學習，我愛java");
});
}
}
運行結果
我愛學習，我愛java
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
public class Thread02 {
public static void main(String[] args) throws Exception {
ExecutorService pool = Executors.newFixedThreadPool(1);
//我們取一個String類型的返回值
Future<String> future = pool.submit(new Callable<String>() {
public String call() throws Exception {
return "我愛java";
}
});
String result = future.get();
System.out.println("線程的運行結果為: " + result);
//我們取一個boolean類型的返回值
Future<Boolean> future2 = pool.submit(new Callable<Boolean>() {
public Boolean call() throws Exception {
return 6 > 7;
}
});
System.out.println("線程的運行結果為：" + future2.get());
//取一個Runnable類型的任務
Future future3 = pool.submit(()->{
System.out.println();
});
}
}
其運行結果為
線程的運行結果為: 我愛java
線程的運行結果為：false
線程的運行結果為：null
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02 線程池執行後先讓其停止有什麼方法呢？通常線程池的停止方法有兩種：shutdown()和shutdownNow()，那麼他們又有什麼區別呢？
1.shutdown
/**
* Initiates an orderly shutdown in which previously submitted
* tasks are executed, but no new tasks will be accepted.
* Invocation has no additional effect if already shut down.
*
* <p>This method does not wait for previously submitted tasks to
* complete execution. Use {@link #awaitTermination awaitTermination}
* to do that.
*
* @throws SecurityException if a security manager exists and
* shutting down this ExecutorService may manipulate
* threads that the caller is not permitted to modify
* because it does not hold {@link
* java.lang.RuntimePermission}{@code ("modifyThread")},
* or the security managers {@code checkAccess} method
* denies access.
*/
void shutdown(); 
觀看其源碼的說明可以知道，shutdown()這種方法是有序的進行停止，在此之前提交的任務都可以繼續執行，而執行此方法後如果繼續往線程池丟任務，則不會再去執行任務。
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class Test01 {
public static void main(String[] args) throws Exception {
ExecutorService pool = Executors.newFixedThreadPool(1);
for (int i = 0; i < 6; i++) {
System.err.println(i);
pool.execute(() -> {
try {
Thread.sleep(1000);
System.out.println("我愛java,我愛學習");
} catch (Exception e) {
e.printStackTrace();
}
});
}
Thread.sleep(2000);
//停止線程池
pool.shutdown();
pool.execute(()->{
System.out.println("我不想上班");
});
}
}
運行結果
0
1
2
3
4
5
我愛java,我愛學習
我愛java,我愛學習
Exception in thread "main" java.util.concurrent.RejectedExecutionException:
Task com.test.Test01$$Lambda$2/2129789493@27d6c5e0 rejected from java.util.concurrent.ThreadPoolExecutor@4f3f5b24[Shutting down,
pool size = 1, active threads = 1, queued tasks = 3, completed tasks = 2]
at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2047)
at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:823)
at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1369)
at com.test.Test01.main(Test01.java:22)
我愛java,我愛學習
我愛java,我愛學習
我愛java,我愛學習
我愛java,我愛學習
通過測試我們先創建一個大小為1的線程池，往裡面丟6個任務，然後調用shutdown()方法去停止線程池，最後我們再去執行一個任務 可以發現我們剛開始放進去的任務都有序的執行了，當我們執行為shutdown()方法後，再去調用線程池，此時線程池已經不工作了。
2.shutdownNow()
/**
* Attempts to stop all actively executing tasks, halts the
* processing of waiting tasks, and returns a list of the tasks
* that were awaiting execution.
*
* <p>This method does not wait for actively executing tasks to
* terminate. Use {@link #awaitTermination awaitTermination} to
* do that.
*
* <p>There are no guarantees beyond best-effort attempts to stop
* processing actively executing tasks. For example, typical
* implementations will cancel via {@link Thread#interrupt}, so any
* task that fails to respond to interrupts may never terminate.
*
* @return list of tasks that never commenced execution
* @throws SecurityException if a security manager exists and
* shutting down this ExecutorService may manipulate
* threads that the caller is not permitted to modify
* because it does not hold {@link
* java.lang.RuntimePermission}{@code ("modifyThread")},
* or the security managers {@code checkAccess} method
* denies access.
*/
List<Runnable> shutdownNow(); 
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class Test01 {
public static void main(String[] args) throws Exception {
ExecutorService pool = Executors.newFixedThreadPool(1);
for (int i = 0; i < 6; i++) {
System.err.println(i);
pool.execute(() -> {
try {
//1秒執行一個
Thread.sleep(1000);
System.out.println("我愛java,我愛學習");
} catch (Exception e) {
e.printStackTrace();
}
});
}
//休眠兩秒
Thread.sleep(2000);
List<Runnable> runnables= pool.shutdownNow();
System.out.println("未執行的線程"+runnables);
System.out.println("未執行的線程的數量"+runnables.size());
}
}
運行結果
0
1
2
3
4
5
我愛java,我愛學習
我愛java,我愛學習
未執行的線程[com.test.Test01$$Lambda$1/931919113@7ef20235, com.test.Test01$$Lambda$1/931919113@7ef20235,
com.test.Test01$$Lambda$1/931919113@7ef20235, com.test.Test01$$Lambda$1/931919113@7ef20235]
未執行的線程的數量4
通過上面代碼演示，我們先放6個任務進如線程池，每秒執行一個，然後休眠2秒，通過運行結果我們可以知道在休眠的2秒鐘執行了兩個線程，所以還有四個線程未執行，輸出結果為4；
當我們把休眠時間變為2500毫秒（2.5秒）其輸出結果為：
0
1
2
3
4
5
我愛java,我愛學習
我愛java,我愛學習
未執行的線程[com.test.Test01$$Lambda$1/931919113@7ef20235, com.test.Test01$$Lambda$1/931919113@7ef20235,
com.test.Test01$$Lambda$1/931919113@7ef20235]
未執行的線程的數量3
java.lang.InterruptedException: sleep interrupted
at java.lang.Thread.sleep(Native Method)
at com.test.Test01.lambda$main$0(Test01.java:14)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
可以看到當我們讓其休眠2.5秒後。在這段時間執行完成了2個線程，第三個線程正在執行中（還未執行完），而shutdownNow()方法是停止所有線程（執行中的和等待執行的），返回正在等待執行的線程。所以其返回正在等待執行的數量為3
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