Android-json解析（二）：Jackson的使用

github链接：Jackson

推荐阅读 : Jackson 工具类使用及配置指南

使用Jackson，主要是使用writeValueAsString和readValue这两个方法。此外，Jackson的writeValue和readValue有很多种参数类型，适用于多中场合。

1、导入依赖

compile 'com.fasterxml.jackson.core:jackson-databind:2.9.0.pr3'
compile 'com.fasterxml.jackson.core:jackson-core:2.9.0.pr3'
compile 'com.fasterxml.jackson.core:jackson-annotations:2.9.0.pr3'

2、添加packagingOptions

因为jackson-core和jackson-databind有重复的文件，重复加载会报错。

android{

...
packagingOptions {
exclude 'META-INF/NOTICE' // will not include NOTICE file
exclude 'META-INF/LICENSE' // will not include LICENSE file
}
}

3、工具类：

package com.example.administrator.gsondemo;

import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
* Created by Administrator on 2017/5/10.
*/

public class JacksonUtil {

private final static ObjectMapper objectMapper = new ObjectMapper();

private JacksonUtil() {

}

public static ObjectMapper getInstance() {

return objectMapper;
}

/**
* javaBean,list,array convert to json string
*/
public static String obj2json(Object obj) throws Exception {
return objectMapper.writeValueAsString(obj);
}

/**
* json string convert to javaBean
*/
public static <T> T json2pojo(String jsonStr, Class<T> clazz)
throws Exception {
return objectMapper.readValue(jsonStr, clazz);
}

/**
* json string convert to map
*/
public static <T> Map<String, Object> json2map(String jsonStr)
throws Exception {
return objectMapper.readValue(jsonStr, Map.class);
}

/**
* json string convert to map with javaBean
*/
public static <T> Map<String, T> json2map(String jsonStr, Class<T> clazz)
throws Exception {
Map<String, Map<String, Object>> map = objectMapper.readValue(jsonStr,
new TypeReference<Map<String, T>>() {
});
Map<String, T> result = new HashMap<String, T>();
for (Map.Entry<String, Map<String, Object>> entry : map.entrySet()) {
result.put(entry.getKey(), map2pojo(entry.getValue(), clazz));
}
return result;
}

/**
* json array string convert to list with javaBean
*/
public static <T> List<T> json2list(String jsonArrayStr, Class<T> clazz)
throws Exception {
List<Map<String, Object>> list = objectMapper.readValue(jsonArrayStr,
new TypeReference<List<T>>() {
});
List<T> result = new ArrayList<T>();
for (Map<String, Object> map : list) {
result.add(map2pojo(map, clazz));
}
return result;
}

/**
* map convert to javaBean
*/
public static <T> T map2pojo(Map map, Class<T> clazz) {
return objectMapper.convertValue(map, clazz);
}

}

4、测试

Person 类：

package com.example.administrator.gsondemo;

/**
* Created by Administrator on 2017/5/9.
*/

public class Person {

private String name;
private int age;
private Birthday birthday;

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public int getAge() {
return age;
}

public void setAge(int age) {
this.age = age;
}

public Birthday getBirthday() {
return birthday;
}

public void setBirthday(Birthday birthday) {
this.birthday = birthday;
}

public Person() {
}

public Person(String name, int age, Birthday birthday) {
this.name = name;
this.age = age;
this.birthday = birthday;
}

@Override
public String toString() {
return "Person{" +
"name='" + name + '\'' +
", age=" + age +
", birthday=" + birthday +
'}';
}
}

Birthday 类：

package com.example.administrator.gsondemo;

/**
* Created by Administrator on 2017/5/9.
*/
public class Birthday {
private int year;
private int month;
private int day;

public int getYear() {
return year;
}

public void setYear(int year) {
this.year = year;
}

public int getMonth() {
return month;
}

public void setMonth(int month) {
this.month = month;
}

public int getDay() {
return day;
}

public void setDay(int day) {
this.day = day;
}

public Birthday() {
}

public Birthday(int year, int month, int day) {
this.year = year;
this.month = month;
this.day = day;
}

@Override
public String toString() {
return "Birthday{" +
"year=" + year +
", month=" + month +
", day=" + day +
'}';
}
}

MainActivity 类：

package com.example.administrator.gsondemo;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;

import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {

private static final String TAG = "MainActivity";

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

objectsToJson_Jackson();
objectToJson_Jackson();
}

/*------------------------------------Jackson----------------------------------*/

/**
* 单一对象的json生成与解析
*/
public void objectToJson_Jackson() {

Person p = new Person("zhaokaiqiang", 22, new Birthday(1992, 1, 19));

String jsonString = null;
try {
jsonString = JacksonUtil.obj2json(p);
} catch (Exception e) {
e.printStackTrace();
}
Log.e(TAG, "---------jackson单一对象生成字符串--------");
Log.e(TAG, jsonString);
Log.e(TAG, jsonString instanceof String ? "类型===String" : "类型不为String");

Person person = null;
try {
person = JacksonUtil.json2pojo(jsonString, Person.class);
} catch (Exception e) {
e.printStackTrace();
}
Log.e(TAG, "---------jackson单一字符串解析成对象--------");
Log.e(TAG, person.toString());
Log.e(TAG, "对象的名字：：：：" + person.getName());
}

/**
* 集合对象的json生成与解析
*/
public void objectsToJson_Jackson() {
Person person1 = new Person("zhaokaiqiang1", 21, new Birthday(1991, 1, 19));
Person person2 = new Person("zhaokaiqiang2", 22, new Birthday(1992, 2, 29));
Person person3 = new Person("zhaokaiqiang3", 23, new Birthday(1993, 3, 39));
ArrayList<Person> arrayList = new ArrayList<Person>();
arrayList.add(person1);
arrayList.add(person2);
arrayList.add(person3);

String jsonString = null;
try {
jsonString = JacksonUtil.obj2json(arrayList);
} catch (Exception e) {
e.printStackTrace();
}
Log.e(TAG, "---------jackson集合对象生成字符串--------");
Log.e(TAG, jsonString);
Log.e(TAG, jsonString instanceof String ? "类型===String" : "类型不为String");

List<Person> persons = null;
try {
persons = JacksonUtil.json2list(jsonString, Person.class);
} catch (Exception e) {
e.printStackTrace();
}
Log.e(TAG, "---------jackson集合字符串解析成对象--------");
Log.e(TAG, persons.toString());
Log.e(TAG, "对象的名字：：：：" + persons.get(0).getName());

}

}

5、问题及解答：

5.1 运行后报以下错误，请参照上面第二条：添加packagingOptions

Error:Execution failed for task ':app:transformResourcesWithMergeJavaResForDebug'.
> com.android.build.api.transform.TransformException: com.android.builder.packaging.DuplicateFileException: Duplicate files copied in APK META-INF/LICENSE
File1: C:\Users\Administrator\.gradle\caches\modules-2\files-2.1\com.fasterxml.jackson.core\jackson-annotations\2.9.0.pr3\dede10be4aaaa6893561372dc5196644268997b7\jackson-annotations-2.9.0.pr3.jar
File2: C:\Users\Administrator\.gradle\caches\modules-2\files-2.1\com.fasterxml.jackson.core\jackson-core\2.9.0.pr3\f9c6d5cfde33c589596d1f43200ec052a2c7c9c0\jackson-core-2.9.0.pr3.jar
File3: C:\Users\Administrator\.gradle\caches\modules-2\files-2.1\com.fasterxml.jackson.core\jackson-databind\2.9.0.pr3\41a8d484e0fe00102724d03090503ad212ee3c20\jackson-databind-2.9.0.pr3.jar

5.2 Gson运行报错

java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to...

异常原因:

作为方法传入参数，Java编译过程会将泛型对象类型擦除。

解决方法：

1、直接copy方法内代码直接使用，而不通过方法传入

2、不使用

public static <T> List<T> getObjectList(String jsonString,Class<T> cls){
List<T> list = new ArrayList<T>();
try {
Gson gson = new Gson();
list = gson.fromJson(jsonString, new TypeToken<List<T>>(){}.getType());
} catch (Exception e) {
e.printStackTrace();
}
return list;
}

而使用以下：

public static <T> List<T> getObjectList(String jsonString,Class<T> cls){
List<T> list = new ArrayList<T>();
try {
Gson gson = new Gson();
JsonArray arry = new JsonParser().parse(jsonString).getAsJsonArray();
for (JsonElement jsonElement : arry) {
list.add(gson.fromJson(jsonElement, cls));
}
} catch (Exception e) {
e.printStackTrace();
}
return list;
}

参考：

java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to

异常com.google.gson.internal.StringMap cannot cast to..原因&解决方案