杰克逊用变量插值反序列化 [英] Jackson deserialize with variable interpolation

问题描述

如何自定义Jackson反序列化包含 $ {token} 的值？

How to custom Jackson to deserialize a value that contains a ${token} ?

这是一个我要在 Apache Commons中添加的功能实例示例配置变量插值：

{
    "file" = "${sys:user.home}/path/to/my_file"
}

推荐答案

您可以根据默认情况注册自定义字符串反序列化程序，这将在反序列化后插入变量。

You can register a custom string deserialiser based on the default which would interpolate the variables after the deserialisation.

但是，正如评论中指出的那样，这对于非String类型（如文件和URL）不起作用。更好的想法是覆盖 JsonParser的 getText（）和 getValueAsString（）方法传递自定义 JsonFactory 。

But, as was pointed out in the comments, that would not work for the non-String types such as File and URLs. The better idea is to override the getText() and getValueAsString() methods of the JsonParser by passing a custom JsonFactory.

以下是一个例子：

public class JacksonInterpolateString {
    static final String JSON = "{ \"file\":\"${sys:user.home}/path/to/my_file\" }";

    public static class Bean {
        public File file;

        @Override
        public String toString() {
            return file.toString();
        }
    }

    private static class MyJsonParser extends JsonParserDelegate {

        public MyJsonParser(final JsonParser d) {
            super(d);
        }

        @Override
        public String getText() throws IOException {
            final String value = super.getText();
            if (value != null) {
                return interpolateString(value);
            }
            return value;
        }

        @Override
        public String getValueAsString() throws IOException {
            return getValueAsString(null);
        }

        @Override
        public String getValueAsString(final String defaultValue) throws IOException {
            final String value = super.getValueAsString(defaultValue);
            if (value != null) {
                return interpolateString(value);
            }
            return null;
        }
    }

    private static class MyMappingJsonFactory extends MappingJsonFactory {
        @Override
        protected JsonParser _createParser(
                final char[] data,
                final int offset,
                final int len,
                final IOContext ctxt,
                final boolean recyclable)
                throws IOException {
            return new MyJsonParser(super._createParser(data, offset, len, ctxt, recyclable));
        }

        @Override
        protected JsonParser _createParser(final Reader r, final IOContext ctxt)
                throws IOException {
            return new MyJsonParser(super._createParser(r, ctxt));
        }

    }

    private static String interpolateString(final String value) {
        return value.replace("${sys:user.home}", "/home/user");
    }

    public static void main(String[] args) throws IOException {
        final JsonFactory factory = new MyMappingJsonFactory();
        final ObjectMapper mapper = new ObjectMapper(factory);
        System.out.println(mapper.readValue(JSON, Map.class));
        System.out.println(mapper.readValue(JSON, Bean.class));
    }

}

输出：

{file=/home/user/path/to/my_file}
/home/user/path/to/my_file

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