使用 Jackson (JSON) 进行序列化 - 获得“未找到序列化程序"? [英] Serializing with Jackson (JSON) - getting "No serializer found"?

问题描述

尝试使用 Jackson 序列化一个非常简单的对象时出现异常.错误:

I get the an exception when trying to serialize a very simple object using Jackson. The error:

org.codehaus.jackson.map.JsonMappingException: 找不到序列化程序类 MyPackage.TestA 并且没有属性发现创建 BeanSerializer(为避免异常，禁用SerializationConfig.Feature.FAIL_ON_EMPTY_BEANS) )

org.codehaus.jackson.map.JsonMappingException: No serializer found for class MyPackage.TestA and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationConfig.Feature.FAIL_ON_EMPTY_BEANS) )

下面是要序列化的简单类和代码.

Below is the simple class and code to serialize.

谁能告诉我为什么我会收到这个错误?

Can anyone tell my why I get this error?

public class TestA {
    String SomeString = "asd";
}

TestA testA = new TestA();
ObjectMapper om = new ObjectMapper();
try {
    String testAString = om.writeValueAsString(testA); // error here!

    TestA newTestA = om.readValue(testAString, TestA.class);
} catch (JsonGenerationException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
} catch (JsonMappingException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
} catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
}

推荐答案

如前所述，ObjectMapper 实例的默认配置是仅访问公共字段或具有公共 getter/setter 的属性.更改类定义以使字段公开或提供公共 getter/setter 的替代方法是指定(对基础 VisibilityChecker)不同的属性可见性规则.Jackson 1.9 为此提供了 ObjectMapper.setVisibility() 方便的方法.对于原始问题中的示例，我可能会将其配置为

As already described, the default configuration of an ObjectMapper instance is to only access properties that are public fields or have public getters/setters. An alternative to changing the class definition to make a field public or to provide a public getter/setter is to specify (to the underlying VisibilityChecker) a different property visibility rule. Jackson 1.9 provides the ObjectMapper.setVisibility() convenience method for doing so. For the example in the original question, I'd likely configure this as

myObjectMapper.setVisibility(JsonMethod.FIELD, Visibility.ANY);

对于杰克逊 >2.0:

For Jackson >2.0:

myObjectMapper.setVisibility(PropertyAccessor.FIELD, Visibility.ANY);

有关相关配置选项的更多信息和详细信息，我建议查看ObjectMapper.setVisibility() 上的 JavaDocs一>.

For more information and details on related configuration options, I recommend reviewing the JavaDocs on ObjectMapper.setVisibility().

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