Java - 主线程

Java 中的主线程

每当我们运行一个 Java 程序时，主线程就会自动创建。这个线程负责执行 Java 程序。Java 运行时会搜索 main 方法来执行，并基于它创建一个主线程。如果我们创建多个线程，那么所有子线程都会从主线程派生。这个主线程是第一个创建的线程，通常也是最后一个线程，它用于执行关闭任务。

如何控制主线程？

当程序启动时，JVM 会自动创建主线程。但你可以使用不同的 Thread 方法和技术来控制主线程。

以下是一些控制主线程的方法。

• sleep() 方法

• join() 方法

• interrupt() 方法

Java 主线程示例

在这个示例中，我们展示了一个简单的单线程程序，没有声明任何线程，并在程序执行中检查线程名称。

package com.;
public class TestThread {
   public void printName() {
	   System.out.println("Thread Name: " + Thread.currentThread().getName());
	   System.out.println("Thread Priority: " +Thread.currentThread().getPriority());
   }	
   public static void main(String args[]) {
	   TestThread thread = new TestThread();
	   thread.printName();	   
   }
}

输出

Thread Name: main
Thread Priority: 5

主线程的更多示例

示例

在这个示例中，我们创建了一个继承自 Thread 类的 ThreadDemo 类。我们没有向 Thread 传递任何名称，它将打印系统分配给线程的默认名称。在 main 方法中，我们创建了两个线程。在输出中，你可以检查到，当前线程名称打印为 main，而线程是通过调用 constructor() 方法创建的。

package com.;
class ThreadDemo extends Thread {
   ThreadDemo( ) {
      System.out.println("Thread: " + Thread.currentThread().getName() + ", " + "State: New");
   }
   public void run() {
      System.out.println("Thread: " + Thread.currentThread().getName() + ", " + "State: Running");
      for(int i = 4; i > 0; i--) {
         System.out.println("Thread: " + Thread.currentThread().getName() + ", " + i); 
      }
      System.out.println("Thread: " + Thread.currentThread().getName() + ", " + "State: Dead");
   }
   public void start () {
      System.out.println("Thread: " + Thread.currentThread().getName() + ", " + "State: Start");
      super.start();
   }
}
public class TestThread {
   public static void main(String args[]) {
	  ThreadDemo thread1 = new ThreadDemo();
	  ThreadDemo thread2 = new ThreadDemo();
	  thread1.start();
	  thread2.start();
   }
}

输出

Thread: main, State: New
Thread: main, State: New
Thread: main, State: Start
Thread: main, State: Start
Thread: Thread-0, State: Running
Thread: Thread-0, 4
Thread: Thread-0, 3
Thread: Thread-1, State: Running
Thread: Thread-1, 4
Thread: Thread-0, 2
Thread: Thread-1, 3
Thread: Thread-0, 1
Thread: Thread-1, 2
Thread: Thread-0, State: Dead
Thread: Thread-1, 1
Thread: Thread-1, State: Dead

示例 2

在这个示例中，我们创建了一个继承自 Thread 类的 ThreadDemo 类。我们没有向 Thread 传递任何名称，它将打印系统分配给线程的默认名称。在 main 方法中，我们创建了两个线程。在输出中，你可以检查到，当前线程名称打印为 main，而线程是通过调用 constructor() 方法创建的。在 main 方法的末尾，我们打印了主线程的状态。

package com.;
class ThreadDemo extends Thread {
   ThreadDemo( ) {
      System.out.println("Thread: " + Thread.currentThread().getName() + ", " + "State: New");
   }
   public void run() {
      System.out.println("Thread: " + Thread.currentThread().getName() + ", " + "State: Running");
      for(int i = 4; i > 0; i--) {
         System.out.println("Thread: " + Thread.currentThread().getName() + ", " + i); 
      }
      System.out.println("Thread: " + Thread.currentThread().getName() + ", " + "State: Dead");
   }
   public void start () {
      System.out.println("Thread: " + Thread.currentThread().getName() + ", " + "State: Start");
      super.start();
   }
}
public class TestThread {
   public static void main(String args[]) {
	  ThreadDemo thread1 = new ThreadDemo();
	  ThreadDemo thread2 = new ThreadDemo();
	  thread1.start();
	  thread2.start();
      System.out.println("Thread: " + Thread.currentThread().getName() + ", " + "State: Dead");
   }
}

输出

Thread: main, State: New
Thread: main, State: New
Thread: main, State: Start
Thread: main, State: Start
Thread: Thread-0, State: Running
Thread: main, State: Dead
Thread: Thread-1, State: Running
Thread: Thread-0, 4
Thread: Thread-1, 4
Thread: Thread-1, 3
Thread: Thread-1, 2
Thread: Thread-1, 1
Thread: Thread-1, State: Dead
Thread: Thread-0, 3
Thread: Thread-0, 2
Thread: Thread-0, 1
Thread: Thread-0, State: Dead

在这个输出中，你可以检查到主线程在早期阶段就结束了，但线程仍在运行并完成了它们的执行。