Java多线程04_线程同步问题

偏执的太偏执、 2022-12-02 10:53 245阅读 0赞

**Java多线程04\_线程同步问题**

**关键字 synchronized 可以保证在同一时刻，只有一个线程可以执行某个方法或某个代码块**

--------------------

**案例1：不安全的买票 (多个线程访问同一个对象的同一个方法)：**

public class ByTicket { 
    	public static void main(String[] args) { 
    		BuyThread buyThread = new BuyThread();
    		new Thread(buyThread,"zhangsan").start();
    		new Thread(buyThread,"lisi").start();
    		new Thread(buyThread,"wangwu").start();
    	}
    }

class BuyThread implements Runnable{ 
    	
    	private int ticketNums = 10;
    	boolean flag = true;
    
    	@Override
    	public void run() { 
    		while(flag) { 
    			try { 
    				buy();
    			} catch (InterruptedException e) { 
    				e.printStackTrace();
    			}
    		}
    	}
    	
    	private void buy() throws InterruptedException { 
    		if(ticketNums<=0) { 
    			flag = false;
    			return;
    		}
    		
    		Thread.sleep(200);
    		
    		System.out.println(Thread.currentThread().getName()+"买到了"+ ticketNums--);
    	}
    	
    }

wangwu买到了10
    zhangsan买到了8
    lisi买到了9
    lisi买到了7
    wangwu买到了7
    zhangsan买到了7
    zhangsan买到了6
    lisi买到了4
    wangwu买到了5
    lisi买到了3
    wangwu买到了3
    zhangsan买到了2
    wangwu买到了1
    lisi买到了0
    zhangsan买到了-1

**解决办法：在 buy() 方法上增加 synchronized 修饰，使其成为同步方法**

private synchronized void buy() throws InterruptedException { 
    	
    }

zhangsan买到了10
    zhangsan买到了9
    zhangsan买到了8
    wangwu买到了7
    wangwu买到了6
    wangwu买到了5
    lisi买到了4
    lisi买到了3
    lisi买到了2
    lisi买到了1

**案例2：不安全的取钱（多个线程操作同一个对象account）：**

public class Bank { 
    	public static void main(String[] args) { 
    		Account account = new Account(100,"存款");
    		DrawMoney blu = new DrawMoney(account,20,"BLU");
    		DrawMoney gf = new DrawMoney(account,100,"gf");
    		
    		blu.start();
    		gf.start();
    	}
    }
    
    class Account{ 
    	int money;
    	String name;
    	
    	public Account(int money, String name) { 
    		super();
    		this.money = money;
    		this.name = name;
    	}
    	
    }
    
    class DrawMoney extends Thread{ 
    	Account account;
    	int draw;
    	int nowmoney;
    	
    	public DrawMoney(Account account, int draw, String name) { 
    		super(name);
    		this.account = account;
    		this.draw = draw;
    		this.nowmoney = nowmoney;
    	}
    	
    	@Override
    	public void run() { 
    		
    		try { 
    			Thread.sleep(1000);
    		} catch (InterruptedException e) { 
    			e.printStackTrace();
    		}
    		
    		if(account.money-draw<0) { 
    			System.out.println(Thread.currentThread().getName()+"账户余额不足");
    			return;
    		}
    		
    		account.money = account.money - draw;
    		nowmoney = nowmoney + draw;
    		System.out.println(account.name+"余额为:"+account.money);
    		System.out.println(Thread.currentThread().getName()+"手里的钱："+nowmoney);
    	}
    	
    }

存款余额为:80
    gf手里的钱：100
    存款余额为:80
    BLU手里的钱：20

**解决办法：使用synchronized (account) \{\} 对给定对象account加锁，进入同步代码块前要获得给定对象的锁**

@Override
    public void run() { 
    	
    	synchronized (account) { 
    		try { 
    			Thread.sleep(1000);
    		} catch (InterruptedException e) { 
    			e.printStackTrace();
    		}
    		
    		if(account.money-draw<0) { 
    			System.out.println(Thread.currentThread().getName()+"账户余额不足");
    			return;
    		}
    		
    		account.money = account.money - draw;
    		nowmoney = nowmoney + draw;
    		System.out.println(account.name+"余额为:"+account.money);
    		System.out.println(Thread.currentThread().getName()+"手里的钱："+nowmoney);
    	}
    
    }

存款余额为:80
    BLU手里的钱：20
    gf账户余额不足

**案例3：不安全的集合（原因是两个线程可能会在同一瞬间操作了同一位置）：**

public class UnsafeList { 
    	
    	public static void main(String[] args) { 
    		List<String> list = new ArrayList<String>();
    		for(int i=0;i<10000;i++) { 
    			new Thread(()->{ 
    				list.add(Thread.currentThread().getName());
    			}).start();
    		}
    		System.out.println(list.size());
    	}
    }

9998

**解决办法：锁住 list 对象**

import java.util.ArrayList;
    import java.util.List;
    
    public class UnsafeList { 
    	
    	public static void main(String[] args) { 
    		List<String> list = new ArrayList<String>();
    		for(int i=0;i<10000;i++) { 
    			new Thread(()->{ 
    				synchronized (list) { 
    					list.add(Thread.currentThread().getName());
    				}
    			}).start();
    		}
    		System.out.println(list.size());
    	}
    }

10000