Java單例模式（懶漢式）線程安全的實現方式

正常的懶漢式實現方式：

public class SingletonDemo {

    private static SingletonDemo instance=null;

    /**
     * 構造方法私有化，防止外部new出來實例對象
     */
    private SingletonDemo(){

    }

    public static SingletonDemo getInstance(){
        if(instance == null){
            instance=new SingletonDemo();
        }
        return instance;
    }

}

線程安全的實現方式：雙重校驗鎖

public class SingletonDemo {

    private volatile static SingletonDemo instance = null;

    /**
     * 構造方法私有化，防止外部new出來實例對象
     */
    private SingletonDemo() {

    }

    public static synchronized SingletonDemo getInstance() {
        if (instance == null) {
            synchronized (SingletonDemo.class) {
                if (instance == null) {
                    instance = new SingletonDemo();
                }
            }
        }
        return instance;
    }

}

案例：利用ThreadLocal設計一個單例模式的線程上下文工具類，應用場景：線程上下文傳參

public class ThreadContextUtil {

    private static ThreadLocal<Map<String,Object>> threadContext = new ThreadLocal<Map<String,Object>>();

    private static volatile  ThreadContextUtil instance;

    /**
     * 構造方法私有化，防止外部new出來實例對象
     */
    private ThreadContextUtil(){

    }

    public static synchronized ThreadContextUtil getInstance() {
        if (instance == null) {
            synchronized (ThreadContextUtil.class) {
                if (instance == null) {
                    instance = new ThreadContextUtil();
                }
            }
        }
        return instance;
    }

    /**
     * 設置鍵值
     * @param key
     * @param object
     */
    public void set(final String key,final Object object) {
        Map<String,Object> map = threadContext.get();
        if (Objects.equal(null, map)) {
            map = Maps.newHashMap();
        }
        map.put(key, object);
        threadContext.set(map);
        return ;
    }

    /**
     * 移除鍵值
     * @param key
     */
    public void removeKey(String key) {
        Map<String,Object> map = threadContext.get();
        if (Objects.equal(null, map)) {
            map = Maps.newHashMap();
        }
        map.remove(key);
        threadContext.set(map);
        return ;
    }

    /**
     * 獲得對應key的值
     * @param key
     * @return
     */
    public Object get(String key) {
        Map<String,Object> map = threadContext.get();
        if (Objects.equal(null, map)) {
            return null;
        } else {
            return map.get(key);
        }
    }

    /**
     * 清理所有
     */
    public void clean() {
        Map<String,Object> map = threadContext.get();
        if (!Objects.equal(null, map)) {
            map.clear();
            threadContext.set(map);
        }
    }
}