# 1.6 Double Check and Lock Singleton

根據1.3節提到的雙重鎖定機制, 這邊附上範例:

```java
package idv.design.pattern.singleton.doublecheck;

/**
 * @author Carl Lu
 */
public class Singleton {

    /*
     * With volatile, the variable value will not be cached by local thread,
     * all the read/write operations are against the shared memory so that
     * we can ensure threads can handle the variable properly.
     *
     * This pattern is also called "DCL".
     */
    private volatile static Singleton instance = null;

    private Singleton() {

    }

    public static Singleton getInstance() {
        // First check
        if (instance == null) {
            synchronized (Singleton.class) {
                // Second check
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }

}
```

原始碼[點我](https://github.com/yotsuba1022/design-pattern/blob/master/src/main/java/idv/design/pattern/singleton/doublecheck/Singleton.java)

其實, 這邊使用volatile背後的意義, 遠比這裡表面上看到的要來得複雜, 我寫在[這邊](https://yotsuba1022.gitbooks.io/about-java-memory-model-jmm-a-k-a-jsr-133/content/1111111.html), 有興趣的話可以看看.


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