大战设计模式（第二季）【2】———— 从源码看单例模式

package com.linkinstars.singleton;

/**
 * （双重检查）单例模式
 * @author LinkinStar
 */
public class DoubleCheckSingleton {
    
    private DoubleCheckSingleton(){}
    
    public volatile static DoubleCheckSingleton doubleCheckSingleton = null;
    
    public static DoubleCheckSingleton getInstance(){
        if (doubleCheckSingleton == null) {
            synchronized(DoubleCheckSingleton.class){
                if (doubleCheckSingleton == null) {
                    doubleCheckSingleton = new DoubleCheckSingleton();
                }
            }
        }
        
        return doubleCheckSingleton;
    }
}

package com.linkinstars.singleton;

/**
 * 饿汉式单例
 * @author LinkinStar
 */
public class HungrySingleton {
    private final static HungrySingleton INSTANCE = new HungrySingleton();

    public static HungrySingleton getInstance() {
        return INSTANCE;
    }

    private HungrySingleton() {
        if (INSTANCE != null) {
            throw new RuntimeException("Can't create singleton by reflect.");
        }
    }
}

package com.linkinstars.singleton;

/**
 * 静态内部类实现单例模式
 * @author LinkinStar
 */
public class StaticInnerClassSingleton {
    private static class InnerClass {
        private static StaticInnerClassSingleton staticInner = new StaticInnerClassSingleton(); 
    }
    
    public StaticInnerClassSingleton getInstance(){
        return InnerClass.staticInner;
    }
    
    private StaticInnerClassSingleton(){}
}

package com.linkinstars.singleton;

/**
 * 枚举实现单例模式
 * @author LinkinStar
 */
public enum EnumSingleton {
    /** 单例 **/
    INSTANCE;
    
    private Object data;

    public Object getData() {
        return data;
    }

    public void setData(Object data) {
        this.data = data;
    }
    
    public static EnumSingleton getInstance() {
        return INSTANCE;
    }
}

package com.linkinstars.singleton;

/**
 * 单线程级别的单例
 * @author LinkinStar
 */
public class ThreadLocalSingleton {
    public static final ThreadLocal<ThreadLocalSingleton> THREAD_LOCAL_SINGLETON_THREAD_LOCAL 
            = ThreadLocal.withInitial(ThreadLocalSingleton::new);
    
    private ThreadLocalSingleton(){
        
    }
    
    public static ThreadLocalSingleton getInstance(){
        return THREAD_LOCAL_SINGLETON_THREAD_LOCAL.get();
    }
}