java - 枚举 Logger 中的 NullpointerException

Question

我有一个 ValueRepository 类，并且想在其中使用记录器。以下代码始终抛出 NullPointerException。

public enum ValueRepository {

INSTANCE;

private static final Logger LOGGER = LoggerFactory.getLogger(ValueRepository.class.getName());

private Map<String, Set<String>> key2value;
private Map value2key;

ValueRepository() {

    key2value = new HashMap();
    value2key = new HashMap();

    load("keyValue.txt");
}

public int size() {
    return key2value.size();
}

public boolean hasKey(String key) {
    return key2value.containsKey(key);
}

public boolean hasValue(String value) {
    return value2key.containsKey(value);
}

public Set<String> getValues(String key) {
    return key2value.get(key);
}

public String getKey(String value) {
    return (String) value2key.get(value);
}

private void load(String filePath) {

    BufferedReader br;
    try {
        br = IOUtils.fileReaderAsResource(filePath);
        String line = null;
        while ((line = br.readLine()) != null) {
            LOGGER.info("test");
            line = line.trim();
        }
        br.close();
    } catch (IOException io) {
        LOGGER.error("Can't load the file: " + filePath);
        io.printStackTrace();
    }
}

public static void main(String[] args) {

        // do nothing
    }

这不应该起作用吗？

这个问题与链接有很大不同。它特别是关于为什么它在枚举中的记录器中不起作用。

编辑:

SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/Users/congmi/.m2/repository/org/apache/logging/log4j/log4j-slf4j-impl/2.6.2/log4j-slf4j-impl-2.6.2.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/Users/congmi/.m2/repository/org/slf4j/slf4j-log4j12/1.7.21/slf4j-log4j12-1.7.21.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.
SLF4J: Actual binding is of type [org.apache.logging.slf4j.Log4jLoggerFactory]
Exception in thread "main" java.lang.ExceptionInInitializerError
Caused by: java.lang.NullPointerException
    at com.qe.repository.ValueRepository.load(ValueRepository.java:56)
    at com.qe.repository.ValueRepository.<init>(ValueRepository.java:26)
    at com.qe.repository.ValueRepository.<clinit>(ValueRepository.java:14)

Process finished with exit code 1

Answer 1

因为您将 ValueRepository 单例设计为 enum你得到了静态常量的初始化序列:

1. INSTANCE(隐式调用构造函数)
2. 记录器

您需要将此初始化顺序更改为:

1. 记录器
2. 实例

您无法使用 enum 到达此序列，因为 Java 语法坚持enum 实例是第一个。

这意味着，您需要将单例实现为普通的类，而不是枚举。

public class ValueRepository {

    private static final Logger LOGGER = LoggerFactory.getLogger(ValueRepository.class.getName());

    public static final ValueRepository INSTANCE = new ValueRepository();

    // remaining code unchanged

}