Java Thread Programming 1.2 – A Simple Two-Thread Example

There are two ways to create a new class that can have a thread running within it. One way is to extend the Thread class. The other is to extend any class and implement the Runnable interface. For the sake of illustration, extending Thread is the simplest approach and is initially used in this book. Implementing the Runnable interface tends to work much better in the real world; this technique is introduced in Chapter 4, “Implementing Runnable Versus Extending Thread.”

兩種建立支援線程類的方式：

• 實現Runnable介面，可繼承自任何類
• 繼承Thread類

顯然第一種方式比較靈活

The steps to spawn a new thread in this chapter’s example are

• Extend the java.lang.Thread class.

• Override the run() method in this subclass of Thread.

• Create an instance of this new class.

• Invoke the start() method on the instance

建立一個線程的步驟：

• 繼承java.lang.Thread類
• 覆蓋子類的run()方法
• 建立一個新類的執行個體
• 調用執行個體的start()方法

A call to start() returns right away and does not wait for the other thread to begin execution. In start(), the parent thread asynchronously signals through the JavaVM that the other thread should be started as soon as it’s convenient for the thread scheduler. At some unpredictable time in the very near future, the other thread will come alive and invoke the run() method of the Thread object (or in this case, the overridden run() method implemented in TwoThread). Meanwhile, the original thread is free to continue executing the statements that follow the start() call.

子父線程非同步運行，調用start()後子線程不確定時間開始執行

Important to note is that although the order in which each thread will execute its own statements is known and straightforward, the order in which the statements will actually be run on the processor is indeterminate, and no particular order should be counted on for program correctness.

線程內順序執行，線程間執行順序不確定

Listing 2.1  TwoThread.java—The Complete Code for the TwoThread Example 1: public class TwoThread extends Thread { 2: public void run() { 3: for ( int i = 0; i < 10; i++ ) { 4: System.out.println(“New thread”); 5: } 6: } 7: 8: public static void main(String[] args) { 9: TwoThread tt = new TwoThread(); 10: tt.start(); 11: 12: for ( int i = 0; i < 10; i++ ) { 13: System.out.println(“Main thread”); 14: } 15: } 16: }