Java多執行緒實作同時輸出

一道經典的面試題目：兩個線程，分別打印AB，其中線程A打印A，線程B打印B，各打印10次，使之出現ABABABABA.. 的效果

package com.shangshe.path;
 
public class ThreadAB {
 
  /**
  * @param args
  */
  public static void main(String[] args) {
     
    final Print business = new Print();
     
    new Thread(new Runnable() {
      public void run() {
        for(int i=0;i<10;i++) {
          business.print_A();
        }
      }
    }).start();
     
    new Thread(new Runnable() {
      public void run() {
        for(int i=0;i<10;i++) {
          business.print_B();
        }
      }
    }).start();
     
  }
}
class Print {
   
  private boolean flag = true;
   
  public synchronized void print_A () {
    while(!flag) {
      try {
        this.wait();
      } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
      }
    }
    System.out.print("A");
    flag = false;
    this.notify();
  }
   
  public synchronized void print_B () {
    while(flag) {
      try {
        this.wait();
      } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
      }
    }
    System.out.print("B");
    flag = true;
    this.notify();
  }
}

由上面的例子我們可以設計出3個線程乃至於n個線程的程序，下面給出的例子是3個線程，分別打印A，B，C 10次，使之出現ABCABC.. 的效果

public class ThreadABC {
 
  /**
   * @param args
   */
  public static void main(String[] args) {
     
    final Print business = new Print();
     
    new Thread(new Runnable() {
      public void run() {
        for(int i=0;i<100;i++) {
          business.print_A();
        }
      }
    }).start();
     
    new Thread(new Runnable() {
      public void run() {
        for(int i=0;i<100;i++) {
          business.print_B();
        }
      }
    }).start();
     
    new Thread(new Runnable() {
      public void run() {
        for(int i=0;i<100;i++) {
          business.print_C();
        }
      }
    }).start();
     
  }
}
class Print {
   
  private boolean should_a = true;
  private boolean should_b = false;
  private boolean should_c = false;
   
  public synchronized void print_A () {
    while(should_b || should_c) {
      try {
        this.wait();
      } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
      }
    }
    System.out.print("A");
    should_a = false;
    should_b = true;
    should_c = false;
    this.notifyAll();
  }
   
  public synchronized void print_B () {
    while(should_a || should_c) {
      try {
        this.wait();
      } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
      }
    }
    System.out.print("B");
    should_a = false;
    should_b = false;
    should_c = true;
    this.notifyAll();
  }
   
  public synchronized void print_C () {
    while(should_a || should_b) {
      try {
        this.wait();
      } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
      }
    }
    System.out.print("C");
    should_a = true;
    should_b = false;
    should_c = false;
    this.notifyAll();
  }
}

再一次證明了軟體工程的重要性了；在多執行緒程式中，應該說在程式中，我們應該把那些業務邏輯程式碼放到同一個類別中，使之高內聚，低耦合

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