并发编程(synchronized)

Java同步机制详解

最新推荐文章于 2025-05-11 18:31:56 发布

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本文深入解析Java中的同步机制，包括synchronized关键字的多种用法，如修饰方法、代码块及静态方法，以及如何通过指定对象作为锁来实现线程同步。通过实例演示了银行账户类的存款和取款操作中如何利用synchronized确保线程安全。

1修饰方法

如下

public synchronized void method(){ // todo}
or 
public void method(){synchronized(this) {// todo }}

2修饰一个代码块

1.一个线程访问一个对象中的synchronized(this)同步代码块时，其他试图访问该对象的线程将被阻塞
2.当一个线程访问对象的一个synchronized(this)同步代码块时，另一个线程仍然可以访问该对象中的非synchronized(this)同步代码块

public  void run() {
	      synchronized(this) {
	         for (int i = 0; i < 5; i++) {
	            try {
	               System.out.println(Thread.currentThread().getName() + ":" + (count++));
	               Thread.sleep(100);
	            } catch (InterruptedException e) {
	               e.printStackTrace();
	            }
	         }
	      }
	   }

3指定要给某个对象加锁

'当有一个明确的对象作为锁时'
/**
 * 银行账户类
 */
class Account {
   String name;
   float amount;
 
   public Account(String name, float amount) {
      this.name = name;
      this.amount = amount;
   }
   //存钱
   public  void deposit(float amt) {
      amount += amt;
      try {
         Thread.sleep(100);
      } catch (InterruptedException e) {
         e.printStackTrace();
      }
   }
   //取钱
   public  void withdraw(float amt) {
      amount -= amt;
      try {
         Thread.sleep(100);
      } catch (InterruptedException e) {
         e.printStackTrace();
      }
   }
 
   public float getBalance() {
      return amount;
   }
}
 
/**
 * 账户操作类
 */
class AccountOperator implements Runnable{
   private Account account;
   public AccountOperator(Account account) {
      this.account = account;
   }
 
   public void run() {
      synchronized (account) {
         account.deposit(500);
         account.withdraw(500);
         System.out.println(Thread.currentThread().getName() + ":" + account.getBalance());
      }
   }
}
 
public class Demo00{
	
	//public static final Object signal = new Object(); // 线程间通信变量
	//将account改为Demo00.signal也能实现线程同步
	public static void main(String args[]){
		Account account = new Account("zhang san", 10000.0f);
		AccountOperator accountOperator = new AccountOperator(account);
 
		final int THREAD_NUM = 5;
		Thread threads[] = new Thread[THREAD_NUM];
		for (int i = 0; i < THREAD_NUM; i ++) {
		   threads[i] = new Thread(accountOperator, "Thread" + i);
		   threads[i].start();
		}
	}
}

"当没有明确的对象作为锁，只是想让一段代码同步时，可以创建一个特殊的对象来充当锁"
class Test implements Runnable
{
   private byte[] lock = new byte[0];  // 特殊的instance变量
   public void method()
   {
      synchronized(lock) {
         // todo 同步代码块
      }
   }
   public void run() {
   }
}

4修饰一个静态的方法

"静态方法是属于类的而不属于对象的。同样的，synchronized修饰的静态方法锁定的是这个类的所有对象"
public synchronized static void method() {
   // todo
}

class SyncThread implements Runnable {
   private static int count;
 
   public SyncThread() {
      count = 0;
   }

   public synchronized static void method() {
      for (int i = 0; i < 5; i ++) {
         try {
            System.out.println(Thread.currentThread().getName() + ":" + (count++));
            Thread.sleep(100);
         } catch (InterruptedException e) {
            e.printStackTrace();
         }
      }
   }
 
   public synchronized void run() {
      method();
   }
}
 
public class Demo00{
	public static void main(String args[]){
		SyncThread syncThread1 = new SyncThread();
		SyncThread syncThread2 = new SyncThread();
		Thread thread1 = new Thread(syncThread1, "SyncThread1");
		Thread thread2 = new Thread(syncThread2, "SyncThread2");
		thread1.start();
		thread2.start();
	}
}

5修饰一个静态的方法

Synchronized还可作用于一个类

class ClassName {
   public void method() {
      synchronized(ClassName.class) {
         // todo
      }
   }
}


/**
 * 同步线程
 */
class SyncThread implements Runnable {
   private static int count;
 
   public SyncThread() {
      count = 0;
   }
   public static void method() {
      synchronized(SyncThread.class) {
         for (int i = 0; i < 5; i ++) {
            try {
               System.out.println(Thread.currentThread().getName() + ":" + (count++));
               Thread.sleep(100);
            } catch (InterruptedException e) {
               e.printStackTrace();
            }
         }
      }
   }
   public synchronized void run() {
      method();
   }
}

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