双重检查解决多线程效率问题

刺骨的言语ヽ痛彻心扉 2022-03-21 11:28 180阅读 0赞

这里以懒汉式创建对象举例  
代码路线：从最简单的单线程--->多线程同步方法（synchronized）--->同步块--->同步块的改进（双重检查）  
简单说明：同步方法和同步块效率大致相同，通过双重检查的模式能最多限度提高效率。

package synchronize;
    /**
     * 单例模型
     * 懒汉式：double checking解决多线程创建对象
     * @author john
     *
     */
    public class SynDemo3 {
    
    	/**
    	 * @param args
    	 */
    	public static void main(String[] args) {
    	/*	//单线程
    		Jvm jvm1=Jvm.getInstance();
    		Jvm jvm2=Jvm.getInstance();
    		System.out.println(jvm1);
    		System.out.println(jvm2);
    		*/
    		JvmThread thread1=new JvmThread(100);
    		JvmThread thread2=new JvmThread(500);
    		thread1.start();
    		thread2.start();
    
    	}
    
    }
    
    
    class JvmThread extends Thread{
    	private long time;
    	public JvmThread(){
    		
    	}
    	public JvmThread(long time){
    		this.time=time;
    	}
    	public void run(){
    		try {
    			System.out.println(Thread.currentThread().getName()+"--->创建"+Jvm.getInstance3(time));
    		} catch (InterruptedException e) {
    			e.printStackTrace();
    		}
    	}
    	
    }
    
    class Jvm{
    	private static Jvm instance =null;
    	//构造器私有化
    	private Jvm(){
    	}
    	public static Jvm getInstance(long time) throws InterruptedException{
    		if(null==instance){
    			Thread.sleep(time); //放大错误
    			instance=new Jvm();
    		}
    		return instance;
    	}
    	
    	
    	//同步方法
    	public static synchronized Jvm getInstance1(long time) throws InterruptedException{
    		if(null==instance){
    			Thread.sleep(time); //放大错误
    			instance=new Jvm();
    		}
    		return instance;
    	}
    	
    	//同步块
    	public static Jvm getInstance2(long time) throws InterruptedException{
    		//效率不高与getInstance1相同，存在对象也需要等待
    		synchronized (Jvm.class) {
    			if(null==instance){
    				Thread.sleep(time); //放大错误
    				instance=new Jvm();
    			}
    			return instance;
    		}
    	}
    	
    	//同步块改进
    	public static Jvm getInstance3(long time) throws InterruptedException{
    		//效率高，提高已经存在对象的访问效率
    		if(null==instance){
    			synchronized (Jvm.class) {
    				if(null==instance){
    					Thread.sleep(time); //放大错误
    					instance=new Jvm();
    				}
    				return instance;
    			}
    		}
    		return instance;
    	}
    }