Guava LoadingCache不能缓存null值

超、凢脫俗 2021-06-22 15:37 542阅读 0赞

测试的时候发现项目中的LoadingCache没有刷新，但是明明调用了refresh方法了。后来发现LoadingCache是不支持缓存null值的，如果load回调方法返回null，则在get的时候会抛出异常。

通过几个例子开看这个问题：

public void test_loadNull() {
        LoadingCache<String, String> stringCache = CacheBuilder.newBuilder()
                .maximumSize(10)
                .build(new CacheLoader<String, String>() {
                    @Override
                    public String load(String s) throws Exception {
                        System.out.println("xx");
                        if (s.equals("hello"))
                            return "world";
                        else
                            return null;
                    }
                });
    
        try {
            System.out.println(stringCache.get("hello"));
    
            // get触发load，load返回null则抛出异常：
            // com.google.common.cache.CacheLoader$InvalidCacheLoadException: CacheLoader returned null for key other_key.
            System.out.println(stringCache.get("other_key"));
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }

public void test_loadNullWhenRefresh() {
        LoadingCache<String, String> stringCache = CacheBuilder.newBuilder()
                .maximumSize(10)
                .build(new CacheLoader<String, String>() {
                    int i = 0;
    
                    @Override
                    public String load(String s) throws Exception {
                        if (i == 0) {
                            i++;
                            return "world";
                        }
                        return null;
                    }
                });
    
        try {
            System.out.println(stringCache.get("hello"));
            System.out.println(stringCache.get("hello"));
    
            // refresh的时候，如果load函数返回null，则refresh抛出异常：
            // Exception thrown during refresh
            // com.google.common.cache.CacheLoader$InvalidCacheLoadException: CacheLoader returned null for key hello.
            stringCache.refresh("hello");
    
            System.out.println(stringCache.get("hello"));
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }

public void test_loadNullAfterInvalidate() {
        LoadingCache<String, String> stringCache = CacheBuilder.newBuilder()
                .maximumSize(10)
                .build(new CacheLoader<String, String>() {
                    int i = 0;
    
                    @Override
                    public String load(String s) throws Exception {
                        if (i == 0) {
                            i++;
                            return "world";
                        }
                        return null;
                    }
                });
    
        try {
            System.out.println(stringCache.get("hello"));
            System.out.println(stringCache.get("hello"));
    
            // invalidate不会触发load
            stringCache.invalidate("hello");
    
            // invalidate后，再次get，触发load，抛出异常：
            // com.google.common.cache.CacheLoader$InvalidCacheLoadException: CacheLoader returned null for key hello.
            System.out.println(stringCache.get("hello"));
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }

public void test_loadThrowException() {
        LoadingCache<String, String> stringCache = CacheBuilder.newBuilder()
                .maximumSize(10)
                .build(new CacheLoader<String, String>() {
                    @Override
                    public String load(String s) throws Exception {
                        if (s.equals("hello"))
                            return "world";
                        else
                            throw new IllegalArgumentException("only_hello");
                    }
                });
    
        try {
            System.out.println(stringCache.get("hello"));
    
            // get触发load，load抛出异常，get也会抛出封装后的异常：
            // com.google.common.util.concurrent.UncheckedExecutionException: java.lang.IllegalArgumentException: only_hello
            System.out.println(stringCache.get("other_key"));
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }

所以如果你需要缓存“空”值，推荐的做法是使用Optional对象来封装结果：

public void test_loadUseOptional() {
        LoadingCache<String, Optional<String>> stringCache = CacheBuilder.newBuilder()
                .maximumSize(10)
                .build(new CacheLoader<String, Optional<String>>() {
                    @Override
                    public Optional<String> load(String s) throws Exception {
                        if (s.equals("hello"))
                            return Optional.of("world");
                        else
                            return Optional.absent();
                    }
                });
    
        try {
            Optional<String> hello = stringCache.get("hello");
            if(hello.isPresent()) {
                System.out.println(hello.get());
            }
    
            Optional<String> otherKey = stringCache.get("other_key");
            if(otherKey.isPresent()){
                System.out.println(otherKey.get());
            }
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }

如果你的场景中认为null是不存在的，那么你可以在load函数中抛出异常，这个异常会通过get抛出。

另外还有一个问题，如果是key==null呢？答案是直接抛出`java.lang.NullPointerException`。Guava对于null是很不待见的。

解决方案：

1.  可以捕获 CacheLoader$InvalidCacheLoadException 这个异常，return null
2.  避免传入缓存null值，使用Optional等类，在key传入之前，就避免掉value

## 参考资料 ##

*  \[Google Guava\] 3-缓存 | 并发编程网 – ifeve.com  
    [http://ifeve.com/google-guava-cachesexplained/][http_ifeve.com_google-guava-cachesexplained]
 *  Guava Cache使用笔记 - 代码说-Let code talk - ITeye博客  
    [http://bylijinnan.iteye.com/blog/2225074][http_bylijinnan.iteye.com_blog_2225074]
 *  guava - How to avoid caching when values are null? - Stack Overflow  
    [https://stackoverflow.com/questions/13379071/how-to-avoid-caching-when-values-are-null][https_stackoverflow.com_questions_13379071_how-to-avoid-caching-when-values-are-null]

[http_ifeve.com_google-guava-cachesexplained]: http://ifeve.com/google-guava-cachesexplained/
[http_bylijinnan.iteye.com_blog_2225074]: http://bylijinnan.iteye.com/blog/2225074
[https_stackoverflow.com_questions_13379071_how-to-avoid-caching-when-values-are-null]: https://stackoverflow.com/questions/13379071/how-to-avoid-caching-when-values-are-null