Java线程同步 线程死锁

　　

 

解决超卖

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package ersatz.thread;   public class T {   public static void main(String[] args) {     Ticket ticket = new Ticket();     new Thread(ticket).start();     new Thread(ticket).start();     new Thread(ticket).start();   } }   class Ticket implements Runnable {   private int number = 50;   private boolean loop = true;   Object o = new Object();     // 同步方法(static)的锁为当前类 //  public synchronized static void m(){} 锁加在Ticket.class //  静态方法中,实现一个同步代码块 //  synchronized (Ticket.class) { // //  }   public static void m() {     synchronized (Ticket.class) {       System.out.println();     }   }     //  public synchronized void m(){}是一个同步方法,锁在this,可在代码块写synchronized,同步代码块,互斥锁还是在this   public /*synchronized*/ void sell() {  // 同步方法,在同一时刻,只能有一个线程来执行sell方法     synchronized (/*this*/ o) {       if (number <= 0) {         System.out.println("ticket out of number");         loop = false;         return;       }       try {         Thread.sleep(50);       } catch (InterruptedException e) {         e.printStackTrace();       }       System.out.println(Thread.currentThread().getName() + " sell ticket " + "left: " + --number);     }   }     @Override   public void run() {     while (loop) sell();   } }

　　

 

以下代码不能解决超卖

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package ersatz.thread;   public class T {   public static void main(String[] args) {     Ticket ticket1 = new Ticket();     Ticket ticket2 = new Ticket();     Ticket ticket3 = new Ticket();     ticket1.start();     ticket2.start();     ticket3.start();   } }   class Ticket extends Thread {   private static int number = 50;   private boolean loop = true;     public void sell() {     synchronized (this) {       if (number <= 0) {         System.out.println("ticket out of number");         loop = false;         return;       }       try {         Thread.sleep(50);       } catch (InterruptedException e) {         e.printStackTrace();       }       System.out.println(Thread.currentThread().getName() + " sell ticket: " + number + ", left: " + --number);     }   }     @Override   public void run() {     while (loop) sell();   } }

　　

 

 

 

 

 

 

Deadlock

package ersatz.thread;

public class T {
  public static void main(String[] args) {
    Deadlock d1 = new Deadlock(true);
    Deadlock d2 = new Deadlock(false);
    d1.setName("Thread-A");
    d2.setName("Thread-B");
    d1.start();
    d2.start();
  }
}

class Deadlock extends Thread {
  protected static final Object o1 = new Object();  // 保证多线程共享同一对象
  protected static final Object o2 = new Object();
  protected boolean flag;

  public Deadlock(boolean flag) {
    this.flag = flag;
  }

  @Override
  public void run() {
    //1. 如果flag 为 T, 线程A 就会先得到/持有 o1 对象锁, 然后尝试去获取 o2 对象锁
    //2. 如果线程A 得不到 o2 对象锁，就会Blocked
    //3. 如果flag 为 F, 线程B 就会先得到/持有 o2 对象锁, 然后尝试去获取 o1 对象锁
    //4. 如果线程B 得不到 o1 对象锁，就会Blocked
    if (flag) {
      synchronized (o1) {  // 对象互斥锁,下面为同步代码
        System.out.println(Thread.currentThread().getName() + " obtain o1 lock");
        synchronized (o2) {
          System.out.println(Thread.currentThread().getName() + " obtain o2 lock");
        }
      }
    } else {
      synchronized (o2) {
        System.out.println(Thread.currentThread().getName() + " obtain o2 lock");
        synchronized (o1) {
          System.out.println(Thread.currentThread().getName() + " obtain o1 lock");
        }
      }
    }
  }
}

 

 

 

 

 

释放锁:

 

 

下列操作不会释放锁

 

 

 

 

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package ersatz.thread;   import java.util.Scanner;   public class T {   public static void main(String[] args) {     A a = new A();     B b = new B(a);     a.setName("Thread-A");     b.setName("Thread-B");     a.start();     b.start();   } }   class A extends Thread {   private boolean loop = true;     public void setLoop(boolean loop) {     this.loop = loop;   }     @Override   public void run() {     while (loop) {       System.out.println((int) (Math.random() * 100 + 1));       try {         Thread.sleep(1000);       } catch (InterruptedException e) {         e.printStackTrace();       }     }     System.out.println(Thread.currentThread().getName() + " exit");   } }   class B extends Thread {   private A a = null;   private final Scanner scanner = new Scanner(System.in);     public B(A a) {     this.a = a;   }     @Override   public void run() {     while (true) {       System.out.println("input Q to exit:");       char key = scanner.next().toUpperCase().charAt(0);       if (key == 'Q') {         a.setLoop(false);         System.out.println(Thread.currentThread().getName() + " exit");         break;       }     }   } }

　　

 

 

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package ersatz.thread; public class T {   public static void main(String[] args) {     B b = new B();     Thread t1 = new Thread(b);     t1.setName("Thread-A");     Thread t2 = new Thread(b);     t2.setName("Thread-B");     t1.start();     t2.start();   } } class B implements Runnable{   private int balance=10000;     @Override   public void run() {     do {       synchronized (this) {         if (balance < 1000) {           System.out.println("insufficient balance: " + balance+' '+Thread.currentThread().getName()+" exit");           break;         }         System.out.println(Thread.currentThread().getName() + " withdraw 1000, balance: " + (balance -= 1000));       }       try {         Thread.sleep(1000);       } catch (InterruptedException e) {         e.printStackTrace();       }     } while (true);   } }