杰克逊：反序列化自定义异常

Question

I am trying to get Jackson to deserialize a JSON-encoded custom exception, and it fails if the exception disables the stacktrace.

Working example:

public static class CustomException extends Exception {
    public CustomException(String msg) {
        super(msg);
    }
}

@Test
public void testSerializeAndDeserializeCustomException() throws Exception {
    log.info("Test: testSerializeAndDeserializeCustomException");

    CustomException ex1 = new CustomException("boom");
    ObjectMapper om = new ObjectMapper();

    String json = om.writerFor(CustomException.class).writeValueAsString(ex1);
    assertNotNull(json);
    log.info("JSON: {}", json);

    CustomException ex2 = om.readerFor(CustomException.class).readValue(json);
    assertNotNull(ex2);
    assertEquals(ex2.getMessage(), ex1.getMessage());
}

Not working example:

public static class CustomNoStackException extends Exception {
    public CustomNoStackException(String msg) {
        super(msg, null, true, false);
    }
}

@Test
public void testSerializeAndDeserializeCustomNoStackException() throws Exception {
    log.info("Test: testSerializeAndDeserializeCustomNoStackException");

    CustomNoStackException ex1 = new CustomNoStackException("boom");
    ObjectMapper om = new ObjectMapper();

    String json = om.writerFor(CustomNoStackException.class).writeValueAsString(ex1);
    assertNotNull(json);
    log.info("JSON: {}", json);

    CustomNoStackException ex2 = om.readerFor(CustomNoStackException.class).readValue(json);
    assertNotNull(ex2);
    assertEquals(ex2.getMessage(), ex1.getMessage());
}

In the second case, readValue(json) actually throws the CustomNoStackException wrapped in an IOException .

What am I doing wrong?

Answer 1

When an Exception is initialized without a cause , it marks itself as the cause .

private Throwable cause = this;

It uses that as a sentinel value to indicate that it has no cause. initCause only lets you change the cause if it's not itself.

Your CustomNoStackException constructor is initializing the cause with null , breaking the sentinel value. When Jackson later tries to call initCause method because of the

"cause":null

pair in the JSON, initCause throws an exception saying you can't overwrite an exception's cause (this is nested in the JsonMappingException ).