在动态JSON中序列化空值和空字符串

Question

我有这个JSON内容:

{"color":null}

我想用它来制作这些Java对象(反之亦然):

Container
|- List<Entry> entries
   |- Entry
      |- String key = "color"
      |- String value = null

我目前的解决方案总是反序列化"color":null到一个空字符串.我找到了其他解决方案,而不是反序列化null或将String清空null.

我怎样才能获得莫西(或任何其他JAXB实现)反序列化null的null空字符串为空字符串？

---

我正在使用此代码:

import java.io.ByteArrayOutputStream;
import java.util.*;

import javax.xml.bind.*;
import javax.xml.bind.annotation.*;

import org.eclipse.persistence.internal.oxm.ByteArraySource;
import org.eclipse.persistence.jaxb.JAXBContextProperties;
import org.eclipse.persistence.oxm.annotations.*;
import org.junit.*;

class Container {

    @XmlVariableNode("key")
    List<Entry> entries = new ArrayList<Entry>();   
}

class Entry {

    @XmlTransient
    public String key;

    @XmlValue
    @XmlNullPolicy(nullRepresentationForXml=XmlMarshalNullRepresentation.XSI_NIL, xsiNilRepresentsNull=false)
    public String value;
}

public class D {

    /** THIS TEST FAILS!!! */

    @Test
    public void unmarshallNull() throws Exception {
        Assert.assertEquals(null, unmarshall("xml", "<root><color xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xsi:nil=\"true\"/></root>"));
        Assert.assertEquals(null, unmarshall("json", "{\"color\":null}"));
    }

    /* All other tests are passing. */

    @Test
    public void unmarshallEmpty() throws Exception {
        Assert.assertEquals("", unmarshall("xml", "<root><color></color></root>"));
        Assert.assertEquals("", unmarshall("json", "{\"color\":\"\"}"));
    }

    @Test
    public void unmarshallValue() throws Exception {
        Assert.assertEquals("red", unmarshall("xml", "<root><color>red</color></root>"));
        Assert.assertEquals("red", unmarshall("json", "{\"color\":\"red\"}"));
    }

    @Test
    public void marshallNull() throws Exception {
        Assert.assertEquals("<color xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xsi:nil=\"true\"/>", marshall("xml", null));
        Assert.assertEquals("{\"color\":null}", marshall("json", null));
    }

    @Test
    public void marshallEmpty() throws Exception {
        Assert.assertEquals("<color></color>", marshall("xml", ""));
        Assert.assertEquals("{\"color\":\"\"}", marshall("json", ""));
    }

    @Test
    public void marshallValue() throws Exception {
        Assert.assertEquals("<color>red</color>", marshall("xml", "red"));
        Assert.assertEquals("{\"color\":\"red\"}", marshall("json", "red"));
    }

    private static String marshall(String format, String value) throws JAXBException {

        // prepare
        JAXBContext jc = JAXBContext.newInstance(Container.class);
        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(JAXBContextProperties.MEDIA_TYPE, "application/"+format);
        marshaller.setProperty(Marshaller.JAXB_FRAGMENT, Boolean.TRUE);
        marshaller.setProperty(JAXBContextProperties.JSON_INCLUDE_ROOT, false);

        // define example data
        Container detail = new Container();
        Entry entry = new Entry();
        entry.key = "color";
        entry.value = value;
        detail.entries.add(entry);

        // marshall
        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
        marshaller.marshal(detail, outputStream);
        return outputStream.toString();
    }

    private static String unmarshall(String format, String raw) throws JAXBException {

        // prepare
        JAXBContext jc = JAXBContext.newInstance(Container.class);
        Unmarshaller unmarshaller = jc.createUnmarshaller();
        unmarshaller.setProperty(JAXBContextProperties.MEDIA_TYPE, "application/"+format);
        unmarshaller.setProperty(JAXBContextProperties.JSON_INCLUDE_ROOT, false);

        // unmarshall
        Container container = unmarshaller.unmarshal(new ByteArraySource(raw.getBytes()), Container.class).getValue();
        return container.entries.get(0).value;
    }
}

它适用于XML但不适用于JSON:

Test failure: unmarshallNull
java.lang.AssertionError: expected:<null> but was:<>

我还将MOXy配置为相关包(jaxb.properties)的jaxb-provider :

javax.xml.bind.context.factory=org.eclipse.persistence.jaxb.JAXBContextFactory

我使用MOXy 2.22.1和Java8(但具有相同的行为2.18,2.19,2.20,2.21,2.22).我的妈妈pom.xml:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>x</groupId>
    <artifactId>x</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <dependencies>
        <dependency>
            <groupId>org.glassfish.jersey.media</groupId>
            <artifactId>jersey-media-moxy</artifactId>
            <version>2.22.1</version>
        </dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

Answer 1

有些事情你可能想尝试.

首先,尝试使用@XmlElement(nillable=true)而不是XmlNullPolicy注释或设置emptyNodeRepresentsNull参数.

其次,你可能想编写自己的东西XmlAdapter,这几乎可以做同样的事情 - 将空字符串unmarshall为null.主要区别在于您还可以手动配置哪些字段将使用适配器解组,哪些不会,因此保留当前行为.请参阅Blaise Doughan的回答,看看如何将自定义连接XmlAdapter到您的配置.

第三,据我所知杰克逊没有这个问题.见这个答案加时赛此Wiki网页有关如何设置自定义解串器的情况下,它的字段.