JAVA中的“ ClassName :: new”与“ new ClassName（）”

时间：2020-06-17 04:16:34

标签： java multithreading

有我的代码：

public class UserThreadFactory implements ThreadFactory {
  private final String namePrefix;
  private final AtomicInteger nextId = new AtomicInteger(1);

  public UserThreadFactory(String whatFeatureOfGroup) {
    this.namePrefix = "From User ThreadFactory's " + whatFeatureOfGroup + "-Worker-";
  }

  @Override
  public Thread newThread(@NotNull Runnable r) {
    String name = namePrefix + nextId.getAndIncrement();
    Thread thread = new Thread(null, r, name, 0, false);
    System.out.println("Thread Name：" + thread.getName() + "\n HashCode：" + r.hashCode());
    return thread;
  }

public class MyRunnableThread implements Runnable {

  public MyRunnableThread() {
    System.out.println("The constructor is called");
  }

  @Override
  public void run() {
    String name = Thread.currentThread().getName();
    try {
      Thread.sleep(100);
      System.out.println(name + hashCode());
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
  }

}

public class CreateThreadByThreadFactory {
  public static void main(String[] args) {
    UserThreadFactory factory = new UserThreadFactory("localHost");
    Thread thread;
    for (int i = 0; i < 20; i++) {
      thread = factory.newThread(new MyRunnableThread());//FIRST WAY
      thread = factory.newThread(() -> new MyRunnableThread());//SECOND WAY
      thread.start();
    }
  }
}

当我使用第一种方法在类CreateThreadByThreadFactory中创建一些therad时，我创建了二十个线程，每个线程都有自己的MyRunnableThread对象。但是，当我使用第二种方法创建therads时，发现它们的MyRunableThread对象的hashCode在所有线程中都是相同的，而MyRunnableThread的构造函数仅被调用一次，为什么？谁能解释发生了什么事？

以下代码是控制台打印的两种方式

FIRST WAY：

The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-1
 HashCode：1996181658
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-2
 HashCode：806353501
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-3
 HashCode：521645586
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-4
 HashCode：1296064247
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-5
 HashCode：1637070917
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-6
 HashCode：780237624
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-7
 HashCode：205797316
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-8
 HashCode：1128032093
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-9
 HashCode：1066516207
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-10
 HashCode：443308702
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-11
 HashCode：935044096
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-12
 HashCode：396180261
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-13
 HashCode：625576447
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-14
 HashCode：1560911714
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-15
 HashCode：939047783
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-16
 HashCode：1237514926
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-17
 HashCode：548246552
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-18
 HashCode：835648992
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-19
 HashCode：1134517053
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-20
 HashCode：492228202
From User ThreadFactory's localHost-Worker-161237514926
From User ThreadFactory's localHost-Worker-17548246552
From User ThreadFactory's localHost-Worker-15939047783
From User ThreadFactory's localHost-Worker-11935044096
From User ThreadFactory's localHost-Worker-10443308702
From User ThreadFactory's localHost-Worker-91066516207
From User ThreadFactory's localHost-Worker-51637070917
From User ThreadFactory's localHost-Worker-41296064247
From User ThreadFactory's localHost-Worker-3521645586
From User ThreadFactory's localHost-Worker-20492228202
From User ThreadFactory's localHost-Worker-18835648992
From User ThreadFactory's localHost-Worker-191134517053
From User ThreadFactory's localHost-Worker-141560911714
From User ThreadFactory's localHost-Worker-6780237624
From User ThreadFactory's localHost-Worker-7205797316
From User ThreadFactory's localHost-Worker-81128032093
From User ThreadFactory's localHost-Worker-12396180261
From User ThreadFactory's localHost-Worker-13625576447
From User ThreadFactory's localHost-Worker-2806353501
From User ThreadFactory's localHost-Worker-11996181658

Process finished with exit code 0

SECOND WAY：

Thread Name：From User ThreadFactory's localHost-Worker-1
 HashCode：1996181658
Thread Name：From User ThreadFactory's localHost-Worker-2
 HashCode：1996181658
Thread Name：From User ThreadFactory's localHost-Worker-3
 HashCode：1996181658
Thread Name：From User ThreadFactory's localHost-Worker-4
 HashCode：1996181658
Thread Name：From User ThreadFactory's localHost-Worker-5
 HashCode：1996181658
Thread Name：From User ThreadFactory's localHost-Worker-6
 HashCode：1996181658
Thread Name：From User ThreadFactory's localHost-Worker-7
 HashCode：1996181658
Thread Name：From User ThreadFactory's localHost-Worker-8
 HashCode：1996181658
Thread Name：From User ThreadFactory's localHost-Worker-9
 HashCode：1996181658
The constructor is called
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-10
 HashCode：1996181658
The constructor is called
The constructor is called
The constructor is called
The constructor is called
The constructor is called
The constructor is called
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-11
 HashCode：1996181658
Thread Name：From User ThreadFactory's localHost-Worker-12
 HashCode：1996181658
The constructor is called
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-13
 HashCode：1996181658
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-14
 HashCode：1996181658
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-15
 HashCode：1996181658
Thread Name：From User ThreadFactory's localHost-Worker-16
 HashCode：1996181658
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-17
 HashCode：1996181658
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-18
 HashCode：1996181658
The constructor is called
The constructor is called
Thread Name：From User ThreadFactory's localHost-Worker-19
 HashCode：1996181658
Thread Name：From User ThreadFactory's localHost-Worker-20
 HashCode：1996181658
The constructor is called
The constructor is called
The constructor is called

Process finished with exit code 0

2 个答案:

答案 0 :(得分：2)

第二行不等同于第一行，它是一个Runnable，它构造MyRunnableThread的实例，但是除非添加“ .run（）”，否则不会调用其run（）。替换为：

thread.start();
thread = factory.newThread(() -> new MyRunnableThread().run());// SECOND WAY

第一个构造MyRunnableThread并让线程调用MyRunnableThread.run（），第二个线程运行lambda构造MyRunnableThread，而MyRunnableThread.run（）调用仅在添加“ .run（）”后运行部分

答案 1 :(得分：1)

谢谢大家，我意识到问题出在哪里。

thread = factory.newThread(MyRunnable::new);

等效

thread = factory.newThread(() -> new MyRunnable());

等效

thread = factory.newThread(new Runnable() {
  @Override
  public void run() {
    new MyRunnable();
  }
});

这与

完全不同 
thread = factory.newThread(new MyRunnable());

问题解决了。

但是，当我使用thread = factory.newThread(() -> new MyRunnable());时，打印的HashCode为什么总是相同？以上三种方法是否相同？ IDEA还建议我可以简化它们。

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