看代码

正常单例：

public class Singleton {
    private static Singleton uniqueInstance;

    private Singleton() {
    }

    public static Singleton getInstance() {
        if (uniqueInstance == null) {
            uniqueInstance = new Singleton();
        }
        return uniqueInstance;
    }
}

多线程需要同步：

public class Singleton {
    private static Singleton uniqueInstance;

    private Singleton() {
    }

    public static synchronized Singleton getInstance() {
        if (uniqueInstance == null) {
            uniqueInstance = new Singleton();
        }
        return uniqueInstance;
    }
}

提前初始化：

public class Singleton {
    private static Singleton uniqueInstance = new Singleton();

    private Singleton() {
    }

    public static Singleton getInstance() {
        return uniqueInstance;
    }
}

双重检查加锁，只会同步一次：

public class Singleton {
    private volatile static Singleton uniqueInstance;

    private Singleton() {
    }

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

singleton pattern 看代码

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