一、ThreadLocal

使用wait/notify方式實現的線程安全，性能將受到很大影響。解決方案是用空間換時間，不用鎖也能實現線程安全。

來看一個小例子，在線程內的set、get就是threadLocal

package thread2;

public class ConnThreadLocal {

    public static ThreadLocal<String> th = new ThreadLocal<String>();
    
    public void setTh(String value){
        th.set(value);
    }
    
 
public void getTh(){
        System.out.println(Thread.currentThread().getName() + ":" + this.th.get());
    }
    
    public static void main(String[] args) throws InterruptedException {
        
        final ConnThreadLocal ct = new ConnThreadLocal();
        Thread t1 = new Thread(new Runnable() {
            @Override
            
 
public void run() {
                ct.setTh("abc");
                ct.getTh();
            }
        }, "t1");
        
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1000);
                    ct.getTh();
                } 
 
catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "t2");
        
        t1.start();
        t2.start();
    }
    
}

二、單例和多線程

單例常見的有饑餓模式和懶漢模式，但是這兩個模式在多線程情況下是不行的。在多線程中考慮到新能和線程安全的問題一般考慮double check instance 和 static inner class這兩種方式實現單例模式。

static inner class方式：

package bhz.base.conn011;

public class Singletion {
    
    private static class InnerSingletion {
        private static Singletion single = new Singletion();
    }
    
    public static Singletion getInstance(){
        return InnerSingletion.single;
    }
    
}

double check instance 方式：

package thread2;

public class DubbleSingleton {

    private static DubbleSingleton ds;
    
    public  static DubbleSingleton getDs(){
        if(ds == null){
            synchronized (DubbleSingleton.class) {
                if(ds == null){
                    ds = new DubbleSingleton();
                }
            }
        }
        return ds;
    }
    
    public static void main(String[] args) {
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(DubbleSingleton.getDs().hashCode());
            }
        },"t1");
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(DubbleSingleton.getDs().hashCode());
            }
        },"t2");
        Thread t3 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(DubbleSingleton.getDs().hashCode());
            }
        },"t3");
        
        t1.start();
        t2.start();
        t3.start();
    }
    
}

線程學習--（六）單例和多線程、ThreadLocal