java - Slf4j with Log4j 在wrapper exception有消息时不打印wrapped exception (caused by)

Question

第一个例子:

public class Main {

    private static final Logger logger = LoggerFactory.getLogger(Main.class);

    public static void main(String[] args) throws Exception {
        try {
            throw new RuntimeException(new NullPointerException("NPE"));
        } catch (RuntimeException e) {
            logger.error("Error:", e);
        }
    }
}

输出:

Error:
java.lang.RuntimeException: java.lang.NullPointerException: NPE
    at Main.main(Main.java:10)

在第二个示例中，我们还向 RuntimeException 添加了一条消息:

throw new RuntimeException("RTE", new NullPointerException("NPE"));

输出:

Error:
java.lang.RuntimeException: RTE
    at Main.main(Main.java:10)

为什么在这种情况下没有记录 NullPointerException？

注意:e.printStackTrace() 在两种情况下都打印两个异常:

java.lang.RuntimeException: RTE
    at Main.main(Main.java:10)
Caused by: java.lang.NullPointerException: NPE
    ... 1 more

版本:

slf4j-api: 1.7.12
slf4j-log4j12: 1.7.12
log4j: 1.2.17

Answer 1

尽可能尝试使用所有文档和调试，我希望这能以任何方式提供帮助:

  @param message the message object to log.
  @param t the exception to log, including its stack trace.
  public void error(Object message, Throwable t) 

所以你的两个案例都包括代码语句抛出的 RuntimeException 的堆栈跟踪。差别不大。

案例 1: throw new RuntimeException(new NullPointerException("NPE"));

引自RuntimeException Java-Doc和 NullPointerException Java-Doc

public RuntimeException(Throwable cause)

Constructs a new runtime exception with the specified cause and a detail message of (cause==null ? null : cause.toString()) (which typically contains the class and detail message of cause). This constructor is useful for runtime exceptions that are little more than wrappers for other throwables.

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public NullPointerException(String s)

Constructs a NullPointerException with the specified detail message.

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所以这可能会回答您问题的第一部分，其中 java.lang.RuntimeException 在执行期间被抛出，这是 由 new NullPointerException 引起的但作为cause==null cause.toString() 的计算结果为 false打印即 java.lang.NullPointerException 现在因为这个异常本身有一条消息传递如下 NPE

注意:您在代码中提到了 NullPointerException 的原因。(因此 cause==null 的计算结果为 false)

案例 2: throw new RuntimeException("RTE", new NullPointerException("NPE"))

public RuntimeException(String message, Throwable cause)

Constructs a new runtime exception with the specified detail message and cause. Note that the detail message associated with cause is not automatically incorporated in this runtime exception's detail message.

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在这种情况下，您最终会得到 java.lang.RuntimeException 被抛出一条消息 RTE 因为你的事业是 RuntimeException 的 child 本身并且父级首先被捕获，它被执行并且在这种情况下未到达子级。