定义

确保某一个类只有一个实例，并且自行实例化，向整个系统提供这个唯一实例

五种单例模式

1.饿汉式

/**
 * gengwei 饿汉式
 */
public class StarvingSingletion {

    private static final StarvingSingletion mInstance = new StarvingSingletion();

    private StarvingSingletion(){

    }

    public static StarvingSingletion getInstance(){
        return mInstance;
    }

    public void printLog(String log){
        Log.e("gengwei",":"+log);
    }
}

使用方式

StarvingSingletion starvingSingletion = StarvingSingletion.getInstance();
starvingSingletion.printLog("饿汉式");

2.懒汉式

/**
 * gengwei 懒汉式
 */
public class SlackerSingletion {
    private static SlackerSingletion mInstance;

    private SlackerSingletion(){

    }

    /**
     * gengwei synchronized线程安全，保证多线程情况下单例的唯一性
     * @return
     */
    public static synchronized SlackerSingletion getInstance(){
        if (mInstance == null) {
            mInstance = new SlackerSingletion();
        }
        return mInstance;
    }

    public void printLog(String log){
        Log.e("gengwei",":"+log);
    }
}

3.双重锁校验机制

/**
 * gengwei 双重校验机制
 */
public class DoubleChackSingletion {
    /**
     * volatile 是为了防止乱序执行导致双重校验机制失效
     */
    private volatile static DoubleChackSingletion mInstance;

    private DoubleChackSingletion(){

    }

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

    public void printLog(String log){
        Log.e("gengwei",":"+log);
    }
}

4.静态内部类

/**
 * gengwei
 * 静态内部类方式
 */
public class InternalClassSingletion {
    private InternalClassSingletion(){

    }

    public static InternalClassSingletion getInstance(){
        return InternalClassSingletionHolder.instance;
    }

    private static class InternalClassSingletionHolder {
        private static final InternalClassSingletion instance = new InternalClassSingletion();
    }

    public void printLog(String log){
        Log.e("gengwei",":"+log);
    }
}

5.枚举

public enum EmumSingletion {
    INSTANCE;
    public void printLog(String log){
        Log.e("gengwei",":"+log);
    }
}

推荐使用双重校验和静态内部类的方式