XStream忽略字段为null的节点如何解决(语言-java)

Question

XStream忽略字段为null的节点如何解决,如何不忽略null的字段

Answer 1

XStream是一个Java库，用于将Java对象序列化为XML，以及将XML反序列化为Java对象。在默认情况下，XStream会忽略null值的字段。如果你想要不忽略null值的字段，可以使用@XStreamNull注解。

下面是一个简单的示例：

import com.thoughtworks.xstream.annotations.XStreamNull;  
  
public class MyObject {  
    private String field1;  
    private String field2;  
  
    @XStreamNull  
    public String getField2() {  
        return field2;  
    }  
  
    // getter and setter methods...  
}

在这个示例中，field2字段被标记为@XStreamNull，这意味着即使它的值为null，它也会被序列化到XML中。

如果你想要全局忽略null值字段，你可以使用XStream.setupDefaultMapper()方法并传递一个自定义的Driver，这个Driver需要实现XStream.ConverterMatcher接口并覆盖isImmutable()方法。例如：

```java
import com.thoughtworks.xstream.XStream;  
import com.thoughtworks.xstream.converters.ConverterMatcher;  
import com.thoughtworks.xstream.converters.DefaultSingleValueConverter;  
import com.thoughtworks.xstream.converters.reflection.ReflectionProvider;  
import com.thoughtworks.xstream.io.xml.XmlFriendlyNameCoder;  
import com.thoughtworks.xstream.mapper.Mapper;  
import com.thoughtworks.xstream.mapper.MapperWrapper;  
import com.thoughtworks.xstream.security.FieldPermission;  
import com.thoughtworks.xstream.security.NullPermission;  
import com.thoughtworks.xstream.security.Permissions;  
import com.thoughtworks.xstream.security.WildcardPermission;  
  
public class MyXStream {  
    public static void setupDefaultMapper() {  
        Permissions defaultPermissions = new Permissions(new FieldPermission[]{new NullPermission()});  
        FieldPermission fieldPermission = new FieldPermission(String[].class, null, null, null);  
        FieldPermission allFieldPermissions = new AllPermission(new FieldPermission[]{fieldPermission});  
        Mapper mapper = new MapperWrapper(new ReflectionProvider(), new XmlFriendlyNameCoder("_-", "_"), defaultPermissions) {  
            @Override  
            public boolean isImmutable(Object source) {  
                return source instanceof String || source instanceof Number || source instanceof Boolean || source instanceof Character || source instanceof Enum || source instanceof Class || source instanceof Interface || source instanceof Enumeration || source instanceof Collection || source instanceof Map || source instanceof Enumeration[_];  
            }  
        };  
        XStream xstream = new XStream(mapper);  
        xstream.registerConverter(new DefaultSingleValueConverter() {  
            @Override  
            public boolean canConvert(@SuppressWarnings("rawtypes") Class type) {  
                return type == String[].class;  
            }  
        });  
        XStream.setDefault(xstream);  
    }  
}

```
在这个示例中，我们覆盖了isImmutable()方法来定义哪些类型的对象是不可变的。我们注册了一个自定义的ConverterMatcher来处理String[]类型的字段。我们还使用Permissions来定义哪些字段是可访问的。最后，我们设置了默认的XStream实例。这样，即使字段的值为null，它也会被序列化到XML中。