gson使用教程-翻译8

原文地址:http://www.studytrails.com/java/json/java-google-json-exclusion-strategy.jsp

在这一节中,我们将会看到如何选择Java对象中的某些属性转换成json.Gson默认是会把java对象中所有的属性都转换成Json.然而，有时候我们想控制那些属性转，那些属性不转．这种控制的方法有几种．甚至对于没有源码的类型都是可以的.不同的方式:
- 使用自定义的注解来忽略注解的属性
- 通过实现ExclusionStrategy接口和实现shouldSkipFiled和shouldSkipClass方法
- 通过使用@Expose注解和在GsonBuilder使用excludeFiledsWithoutExposeAnnotation().这样就会忽略所有没有使用＠Expose注解的属性．

以下的demo展示这三种方式

package com.studytrails.json.gson;import java.awt.Color;import com.google.gson.Gson;import com.google.gson.GsonBuilder;public class ExclusionExample {    public static void main(String[] args) {        // We create an instance of type CAT.        Cat cat = new Cat();        cat.setName("Cat");        cat.setAge(1);        cat.setColor(Color.BLACK);        cat.setCountry("US");        // we allow serializing null. therefore although the fields lazy is        // null, it will be serialized. We add a CustomExclusionStrategy that        // will exclude the Color class. We also allow only those fields that        // have been exposed using the @Expore annotation        Gson gson = new GsonBuilder().serializeNulls().setExclusionStrategies(new CustomExclusionStrategy(Color.class))                .excludeFieldsWithoutExposeAnnotation().create();        System.out.println(gson.toJson(cat));        // prints {"name":"Cat","lazy":null}    }}

Cat类

package com.studytrails.json.gson;import java.awt.Color;import com.google.gson.annotations.Expose;public class Cat {    @Expose    private String name;    private int age;    private Color color;    @Expose    @Country    private String country;    @Expose    private Boolean lazy = null;    public void setAge(int age) {        this.age = age;    }    public void setName(String name) {        this.name = name;    }    public void setColor(Color color) {        this.color = color;    }    public int getAge() {        return age;    }    public String getName() {        return name;    }    public Color getColor() {        return color;    }    public void setCountry(String country) {        this.country = country;    }    public String getCountry() {        return country;    }    public void setLazy(Boolean lazy) {        this.lazy = lazy;    }    public Boolean getLazy() {        return lazy;    }}

Exclusion Strategy

package com.studytrails.json.gson;import com.google.gson.ExclusionStrategy;import com.google.gson.FieldAttributes;/** * This class defines custom exclusion policy. We want to ignore all fields that * have been annotated with the Country annotation. Note that we can also ignore * fields based on name or type. This same policy can be applied to any class. * In this example we apply to the CAT class, but it is not limited to the cat * class. * */public class CustomExclusionStrategy implements ExclusionStrategy {    private Class classToExclude;    public CustomExclusionStrategy(Class classToExclude) {        this.classToExclude = classToExclude;    }    // This method is called for all fields. if the method returns false the    // field is excluded from serialization    @Override    public boolean shouldSkipField(FieldAttributes f) {        if (f.getAnnotation(Country.class) == null)            return false;        return true;    }    // This method is called for all classes. If the method returns false the    // class is excluded.    @Override    public boolean shouldSkipClass(Class<?> clazz) {        if (clazz.equals(classToExclude))            return true;        return false;    }}