測試的時候發(fā)現(xiàn)項目中的LoadingCache沒有刷新，但是明明調(diào)用了refresh方法了。后來發(fā)現(xiàn)LoadingCache是不支持緩存null值的，如果load回調(diào)方法返回null，則在get的時候會拋出異常。

通過幾個例子開看這個問題：

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觸發(fā)load，load返回null則拋出異常：
        // 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的時候，如果load函數(shù)返回null，則refresh拋出異常：
        // 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不會觸發(fā)load
        stringCache.invalidate("hello");

        // invalidate后，再次get，觸發(fā)load，拋出異常：
        // 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觸發(fā)load，load拋出異常，get也會拋出封裝后的異常：
        // com.google.common.util.concurrent.UncheckedExecutionException: java.lang.IllegalArgumentException: only_hello
        System.out.println(stringCache.get("other_key"));
    } catch (ExecutionException e) {
        e.printStackTrace();
    }
}

所以如果你需要緩存“空”值，推薦的做法是使用Optional對象來封裝結(jié)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();
    }
}

如果你的場景中認(rèn)為null是不存在的，那么你可以在load函數(shù)中拋出異常，這個異常會通過get拋出。

另外還有一個問題，如果是key==null呢？答案是直接拋出java.lang.NullPointerException。Guava對于null是很不待見的。

參考資料

• [Google Guava] 3-緩存 | 并發(fā)編程網(wǎng) – ifeve.com
  http://ifeve.com/google-guava-cachesexplained/
• Guava Cache使用筆記 - 代碼說-Let code talk - ITeye博客
  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

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