android jackson解析，对象转json，json转对象，map...

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题外话：以前一直用的Gson，后来发现Jackson的确要比Gson快，现在估计很长一段时间都要用jackson了。

前言以及准备工作：

当我在网上找Jackson的依赖包的时候，看到很多文章都是复制过来复制过去的，里面给的依赖包只有核心包jackson-core-2.x.x，然而代码里面却需要用到jackson-databind-2.x.x里面的东西，比如里面有一个ObjectMapper类，只有后者的依赖包里面才有，所以说，请那些抄博客的人能够copy得有水平一些吗，最基本的依赖包都不讲清楚。当然最好的方法还是学好英语，直接看官方的说明

所以我这里直接直接给出所有jar包的下载地址：http://repo1.maven.org/maven2/com/fasterxml/jackson/

jar包在core文件夹里面，三个版本都下载，并放到AndoidStudio的libs文件夹里面依赖，我这里用的是2.8.5版本：jackson-annotations-2.8.5，jackson-core-2.8.5，jackson-databind-2.8.5。

然后，当你依赖了这三个jar包之后，写好代码运行的时候，十有八九会抛出DuplicateFileException 这个异常，应该是属于文件冲突，有重复的文件之类的，我也没去仔细研究过，解决方法就是：在build.gradle文件的android{}里面添加以下代码

 packagingOptions {        exclude 'META-INF/DEPENDENCIES.txt'        exclude 'META-INF/LICENSE.txt'        exclude 'META-INF/NOTICE.txt'        exclude 'META-INF/NOTICE'        exclude 'META-INF/LICENSE'        exclude 'META-INF/DEPENDENCIES'        exclude 'META-INF/notice.txt'        exclude 'META-INF/license.txt'        exclude 'META-INF/dependencies.txt'        exclude 'META-INF/LGPL2.1'    }

jackson的使用

在这里我只介绍几种常用的方法，如果有更多需求请自行研究

1、java对象转json

Student类：

public class Student {        private String name;        private String birthDay;        private String gender;        public Student(String name, String birthDay, String gender) {            this.name = name;            this.birthDay = birthDay;            this.gender = gender;        }        public String getName() {            return name;        }        public void setName(String name) {            this.name = name;        }        public String getBirthDay() {            return birthDay;        }        public void setBirthDay(String birthDay) {            this.birthDay = birthDay;        }        public String getGender() {            return gender;        }        public void setGender(String gender) {            this.gender = gender;        }    }

代码：

    private void objectToJson() {        Student student = new Student("小智", "2006-12-12", "男");        //ObjectMapper：操作json的核心类        ObjectMapper mapper = new ObjectMapper();        String studentJson = null;        try {            //将java对象转换成json字符串            studentJson = mapper.writeValueAsString(student);        } catch (JsonProcessingException e) {            e.printStackTrace();        }        Log.e("student", studentJson);    }

打印结果：

student: {"birthDay":"2006-12-12","gender":"男","name":"小智"}

2、List/Map集合转换json

代码：

private void listToJson() {        List<Student> list = new ArrayList<>();        list.add(new Student("小智", "2006-12-12", "男"));        list.add(new Student("小刚", "2001-7-13", "男"));        list.add(new Student("小霞", "2006-9-2", "女"));        ObjectMapper mapper = new ObjectMapper();        String studentList = null;        try {            studentList = mapper.writeValueAsString(list);        } catch (JsonProcessingException e) {            e.printStackTrace();        }        Log.e("list", studentList);    }    private void mapToJson() {        Map<String, String> map = new HashMap<>();        map.put("game", "pokemon");        map.put("indie_game", "FireWatch");        map.put("Blizzard", "OverWatch");        ObjectMapper mapper = new ObjectMapper();        String gameMap = null;        try {            gameMap = mapper.writeValueAsString(map);        } catch (JsonProcessingException e) {            e.printStackTrace();        }        Log.e("gameMap", gameMap);    }

打印结果：

list: [{"birthDay":"2006-12-12","gender":"男","name":"小智"},{"birthDay":"2001-7-13","gender":"男","name":"小刚"},{"birthDay":"2006-9-2","gender":"女","name":"小霞"}]gameMap: {"game":"pokemon","Blizzard":"OverWatch","indie_game":"FireWatch"}

3、json转换java对象

代码：

   private void jsonToObject() {        String student = "{\"birthDay\":\"2006-12-12\",\"gender\":\"男\",\"name\":\"小智\"}";        ObjectMapper mapper = new ObjectMapper();        Student student1 = null;        try {            student1 = mapper.readValue(student, Student.class);        } catch (IOException e) {            e.printStackTrace();        }        Log.e("student1", student1.getBirthDay());    }

打印结果：

student1: 2006-12-12

4、将json字符串转换成List集合

代码：

private void jsonToList() {        String studentList = "[{\"birthDay\":\"2006-12-12\",\"gender\":\"男\",\"name\":\"小智\"},{\"birthDay\":\"2001-7-13\",\"gender\":\"男\",\"name\":\"小刚\"}, {\"birthDay\":\"2006-9-2\",\"gender\":\"女\",\"name\":\"小霞\"}]";        ObjectMapper mapper = new ObjectMapper();        List<Student> list = null;        //先要先反序列化复杂类型        //利用ObjectMapper的getTypeFactory().constructParametricType(Class<?> parametrized, Class<?>... parameterClasses)方法        //获取泛型容器(比如List/Map)的type        JavaType javaType = mapper.getTypeFactory().constructParametricType(List.class, Student.class);        try {            //再将这个type作为转换的目标type            list = mapper.readValue(studentList,javaType);        } catch (IOException e) {            e.printStackTrace();        }        Log.e("list", list.get(1).getBirthDay());    }

附上constructParametricType(Class<?> parametrized, Class<?>... parameterClasses)方法的源码

其中parametrized是指你转换的Collection类型，比如List，然后parameterClasses是指参数类型的数组，比如这里的Student

打印结果：

list: 2001-7-13

5、将json字符串转换成Map集合

代码：

 private void jsonToMap() {        String mapString = "{\"birthDay\":\"2006-12-12\",\"gender\":\"男\",\"name\":\"小智\"}";        ObjectMapper mapper = new ObjectMapper();        Map<String, Object> map = null;        //和List一样，Map依旧需要反序列化        JavaType javaType = mapper.getTypeFactory().constructParametricType(HashMap.class, String.class, Object.class);        try {            //JSONObject jsonObject = new JSONObject(messageMap);            map = mapper.readValue(mapString, javaType);        } catch (IOException e) {            e.printStackTrace();        }        if (map != null) {            Log.e("map", map.toString());        }    }
打印结果：

map: {name=小智, birthDay=2006-12-12, gender=男}

以上则是我认为比较常用的一些解析，更复杂的json解析也能用上面的几种方法进行组合解析。

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