java - Play Framework Cache Api 注入(inject)在单例中返回空指针异常

Question

我有一个 Play Framework 应用程序。我正在尝试通过静态方法访问缓存。我决定将缓存包装成单例，但当尝试访问类 CacheSingleton 中的缓存变量时，出现 NullPointerException。问题如何解决？谢谢。

import javax.inject.*;
import play.cache.*;

@Singleton
public final class CacheSingleton {
    @Inject CacheApi cache;
    private static volatile CacheSingleton instance = null;


    private CacheSingleton() {
    }

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

public class CustomLabels {
    public static String get() {
        CacheSingleton tmp = CacheSingleton.getInstance();
        try
        {
            tmp.cache.set("key", "value");
        }catch(Exception e){}
    }
}

Answer 1

不要使用静态注入(inject)。

“一般来说”这是不可能的: https://stackoverflow.com/a/22068572/1118419

Play 使用 Guice 进行 DI，Guice 可以进行静态注入(inject)，但强烈不推荐这种能力:

https://github.com/google/guice/wiki/Injections#static-injections

use CacheApi in Play的正确方法:

import play.cache.*;
import play.mvc.*;

import javax.inject.Inject;

public class Application extends Controller {

    private CacheApi cache;

    @Inject
    public Application(CacheApi cache) {
        this.cache = cache;
    }

    // ...
}