用Jackson进行JSON序列化/反序列化操作
先放出来本文需要用到的类和JSON文件
Speaker类：
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class Speaker {
private int id;
private int age;
private String fullName;
private List<String> tags = new ArrayList<>();
private boolean registered;
public Speaker() {
super();
}
public Speaker(int id, int age, String fullName, List<String> tags, boolean registered) {
super();
this.id = id;
this.age = age;
this.fullName = fullName;
this.tags = tags;
this.registered = registered;
}
public Speaker(int id, int age, String fullName, String[] tags, boolean registered) {
this(id, age, fullName, Arrays.asList(tags), registered);
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getFullName() {
return fullName;
}
public void setFullName(String fullName) {
this.fullName = fullName;
}
public List<String> getTags() {
return tags;
}
public void setTags(List<String> tags) {
this.tags = tags;
}
public boolean isRegistered() {
return registered;
}
public void setRegistered(boolean registered) {
this.registered = registered;
}
@Override
public String toString() {
return String.format("Speaker [id=%s, age=%s, fullName=%s, tags=%s, registered=%s]", id, age, fullName, tags,
registered);
}
}
JSON文件
speaker.json
{
"id": 1,
"fullName": "Larson Richard",
"tags": [
"JavaScript", "AngularJS", "Yeoman"
],
"age": 39,
"registered": true
}
speakers.json
{
"speakers": [
{
"id": 1,
"fullName": "Larson Richard",
"tags": [
"JavaScript",
"AngularJS",
"Yeoman"
],
"age": 39,
"registered": true
},
{
"id": 2,
"fullName": "Ester Clements",
"tags": [
"REST",
"Ruby on Rails",
"APIs"
],
"age": 29,
"registered": true
},
{
"id": 3,
"fullName": "Christensen Fisher",
"tags": [
"Java",
"Spring",
"Maven",
"REST"
],
"age": 45,
"registered": false
}
]
}
1.对Java简单数据类型进行序列化/反序列化操作
这里的简单数据类型是指：
- 整数型
- 字符串
- 数组
- 布尔值
下面的单元测试程序应用了Jackson和JUnit4对Java中的简单数据类型进行序列化/反序列化操作：
import static org.junit.Assert.*;
import java.io.*;
import java.util.*;
import org.junit.Test;
import com.fasterxml.jackson.core.*;
import com.fasterxml.jackson.core.type.*;
import com.fasterxml.jackson.databind.*;
public class BasicJsonTypesTest {
private static final String TEST_SPEAKER = "age = 39\n" +
"fullName = \"Larson Richard\"\n" +
"tags = [\"JavaScript\",\"AngularJS\",\"Yeoman\"]\n" +
"registered = true";
@Test
public void serializeBasicTypes() {
try {
ObjectMapper mapper = new ObjectMapper();
Writer writer = new StringWriter();
int age = 39;
String fullName = new String("Larson Richard");
List<String> tags = new ArrayList<String>(
Arrays.asList("JavaScript", "AngularJS", "Yeoman"));
boolean registered = true;
String speaker = null;
writer.write("age = ");
mapper.writeValue(writer, age);
writer.write("\nfullName = ");
mapper.writeValue(writer, fullName);
writer.write("\ntags = ");
mapper.writeValue(writer, tags);
writer.write("\nregistered = ");
mapper.configure(SerializationFeature.INDENT_OUTPUT, true);
mapper.writeValue(writer, registered);
speaker = writer.toString();
System.out.println(speaker);
assertTrue(TEST_SPEAKER.equals(speaker));
assertTrue(true);
} catch (JsonGenerationException jge) {
jge.printStackTrace();
fail(jge.getMessage());
} catch (JsonMappingException jme) {
jme.printStackTrace();
fail(jme.getMessage());
} catch (IOException ioe) {
ioe.printStackTrace();
fail(ioe.getMessage());
}
}
@Test
public void deSerializeBasicTypes() {
try {
String ageJson = "{ \"age\": 39 }";
ObjectMapper mapper = new ObjectMapper();
Map<String, Integer> ageMap = mapper.readValue(ageJson,
new TypeReference<HashMap<String,Integer>>() {});
Integer age = ageMap.get("age");
System.out.println("age = " + age + "\n\n\n");
assertEquals(39, age.intValue());
assertTrue(true);
} catch (JsonMappingException jme) {
jme.printStackTrace();
fail(jme.getMessage());
} catch (IOException ioe) {
ioe.printStackTrace();
fail(ioe.getMessage());
}
}
}
在上面的实例中，由于@Test注解的声明，JUnit会将serializeBasicTypes()和deSerializeBasicTypes()方法作为测试的一部分来运行。对于JSON数据自身来说，这些单元测试用例并未执行太多断言操作。
归纳一下，以下是Jackson中用于JSON序列化/反序列化的一些重要的类和方法：
- ObjectMapper 类负责在Java和JSON之间进行相互转换。
- ObjectMapper.writeValue() 方法负责将Java数据类型转换为JSON（本例中转换结果被输出到Writer对象中）。
- ObjectMapper.readValue() 方法负责阿静JSON转换为Java数据结构。
2.对Java对象进行序列化/反序列化操作
如果只是对简单数据类型进行序列化/反序列化，并没有什么有趣的功能。下面是对Java对象进行的操作，这时序列化/反序列化显得比较有用。
下面的程序展示了如何用Jackson来序列化/反序列化speaker对象，同时也展示了如何将JSON文件反序列化为多个speaker对象：
import static org.junit.Assert.*;
import java.io.*;
import java.net.*;
import java.util.*;
import org.junit.Test;
import com.fasterxml.jackson.core.*;
import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.databind.type.*;
public class SpeakerJsonFlatFileTest {
private static final String SPEAKER_JSON_FILE_NAME = "speaker.json";
private static final String SPEAKERS_JSON_FILE_NAME = "speakers.json";
private static final String TEST_SPEAKER_JSON = "{\n" +
" \"id\" : 1,\n" +
" \"age\" : 39,\n" +
" \"fullName\" : \"Larson Richard\",\n" +
" \"tags\" : [ \"JavaScript\", \"AngularJS\", \"Yeoman\" ],\n" +
" \"registered\" : true\n" +
"}";
@Test
public void serializeObject() {
try {
ObjectMapper mapper = new ObjectMapper();
String[] tags = {"JavaScript", "AngularJS", "Yeoman"};
Speaker speaker = new Speaker(1, 39, "Larson Richard", tags, true);
String speakerStr = null;
mapper.configure(SerializationFeature.INDENT_OUTPUT, true);
speakerStr = mapper.writeValueAsString(speaker);
System.out.println(speakerStr);
assertTrue(TEST_SPEAKER_JSON.equals(speakerStr));
assertTrue(true);
} catch (JsonGenerationException jge) {
jge.printStackTrace();
fail(jge.getMessage());
} catch (JsonMappingException jme) {
jme.printStackTrace();
fail(jme.getMessage());
} catch (IOException ioe) {
ioe.printStackTrace();
fail(ioe.getMessage());
}
}
private File getSpeakerFile(String speakerFileName) throws URISyntaxException {
ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
URL fileUrl = classLoader.getResource(speakerFileName);
URI fileUri = new URI(fileUrl.toString());
File speakerFile = new File(fileUri);
return speakerFile;
}
@Test
public void deSerializeObject() {
try {
ObjectMapper mapper = new ObjectMapper();
File speakerFile = getSpeakerFile(
SpeakerJsonFlatFileTest.SPEAKER_JSON_FILE_NAME);
Speaker speaker = mapper.readValue(speakerFile, Speaker.class);
System.out.println("\n" + speaker + "\n");
assertEquals("Larson Richard", speaker.getFullName());
assertEquals(39, speaker.getAge());
assertTrue(true);
} catch (URISyntaxException use) {
use.printStackTrace();
fail(use.getMessage());
} catch (JsonParseException jpe) {
jpe.printStackTrace();
fail(jpe.getMessage());
} catch (JsonMappingException jme) {
jme.printStackTrace();
fail(jme.getMessage());
} catch (IOException ioe) {
ioe.printStackTrace();
fail(ioe.getMessage());
}
}
@Test
public void deSerializeMultipleObjects() {
try {
ObjectMapper mapper = new ObjectMapper();
File speakersFile = getSpeakerFile(
SpeakerJsonFlatFileTest.SPEAKERS_JSON_FILE_NAME);
JsonNode arrNode = mapper.readTree(speakersFile).get("speakers");
List<Speaker> speakers = new ArrayList<Speaker>();
if (arrNode.isArray()) {
for (JsonNode objNode : arrNode) {
System.out.println(objNode);
speakers.add(mapper.convertValue(objNode, Speaker.class));
}
}
assertEquals(3, speakers.size());
System.out.println("\n\n\nAll Speakers\n");
for (Speaker speaker: speakers) {
System.out.println(speaker);
}
System.out.println("\n");
Speaker speaker3 = speakers.get(2);
assertEquals("Christensen Fisher", speaker3.getFullName());
assertEquals(45, speaker3.getAge());
assertTrue(true);
} catch (URISyntaxException use) {
use.printStackTrace();
fail(use.getMessage());
} catch (JsonParseException jpe) {
jpe.printStackTrace();
fail(jpe.getMessage());
} catch (JsonMappingException jme) {
jme.printStackTrace();
fail(jme.getMessage());
} catch (IOException ioe) {
ioe.printStackTrace();
fail(ioe.getMessage());
}
}
}
之前的测试用例均使用了JUnit中的断言方法来测试JSON序列化/反序列化的结果。
对于以上的JUnit单元测试，以下几点值得一提：
- serializeObject()方法创建了一个Speaker对象，然后使用ObjectMapper.writeValueAsString()和System.out.println()将其序列化并打印到标准输出。测试代码将SerializationFeature、INDENT_OUTPUT设置为true，以优化JSON输出中的缩进和显示。
- deSerializeObject()方法调用getSpeakerFile()来读取包含speaker对象的JSON输入文件，然后使用ObjectMapper.readValue()将其反序列化为SpeakerJava对象。
- deSerializeMultipleObjects()方法执行了以下操作
- 调用getSpeakerFile()来读取包含speaker对象数组的JSON输入文件。
- 调用ObjectMapper.readTree()来获取JsonNode对象，该对象指向文件中的JSON文档的根结点。
- 访问JSON树中的每个结点，并使用ObjectMapper.convertValue()方法将结点中的speaker数据反序列化为Java中的Speaker对象。
- 打印列表中的所有Speaker对象。
- getSpeakerFile()方法会查找类路径中相应的文件，并执行以下操作：
- 从当前执行线程获取ContextClassLoader对象。
- 使用ClassLoader.getResource()方法从当前类路径中查找相关文件资源。
- 根据文件名的URI创建FIle对象。