一、入門
Jackson中有個ObjectMapper類很是實用，用於Java對象與JSON的互換。
1.JAVA對象轉JSON[JSON序列化]
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import
java.io.IOException;
import
java.text.ParseException;
import
java.text.SimpleDateFormat;
import
com.fasterxml.jackson.databind.ObjectMapper;
public
class
JacksonDemo {
public
static
void
main(String[] args)
throws
ParseException, IOException {
User user =
new
User();
user.setName(
"小民"
);
user.setEmail(
"xiaomin@sina.com"
);
user.setAge(
20
);
SimpleDateFormat dateformat =
new
SimpleDateFormat(
"yyyy-MM-dd"
);
user.setBirthday(dateformat.parse(
"1996-10-01"
));
/**
* ObjectMapper是JSON操作的核心，Jackson的所有JSON操作都是在ObjectMapper中實現。
* ObjectMapper有多個JSON序列化的方法，可以把JSON字符串保存File、OutputStream等不同的介質中。
* writeValue(File arg0, Object arg1)把arg1轉成json序列，並保存到arg0文件中。
* writeValue(OutputStream arg0, Object arg1)把arg1轉成json序列，並保存到arg0輸出流中。
* writeValueAsBytes(Object arg0)把arg0轉成json序列，並把結果輸出成字節數組。
* writeValueAsString(Object arg0)把arg0轉成json序列，並把結果輸出成字符串。
*/
ObjectMapper mapper =
new
ObjectMapper();
//User類轉JSON
//輸出結果：{"name":"小民","age":20,"birthday":844099200000,"email":"xiaomin@sina.com"}
String json = mapper.writeValueAsString(user);
System.out.println(json);
//Java集合轉JSON
//輸出結果：[{"name":"小民","age":20,"birthday":844099200000,"email":"xiaomin@sina.com"}]
List<User> users =
new
ArrayList<User>();
users.add(user);
String jsonlist = mapper.writeValueAsString(users);
System.out.println(jsonlist);
}
}
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2.JSON轉Java類[JSON反序列化]
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import
java.io.IOException;
import
java.text.ParseException;
import
com.fasterxml.jackson.databind.ObjectMapper;
public
class
JacksonDemo {
public
static
void
main(String[] args)
throws
ParseException, IOException {
String json =
"{\"name\":\"小民\",\"age\":20,\"birthday\":844099200000,\"email\":\"xiaomin@sina.com\"}"
;
/**
* ObjectMapper支持從byte[]、File、InputStream、字符串等數據的JSON反序列化。
*/
ObjectMapper mapper =
new
ObjectMapper();
User user = mapper.readValue(json, User.
class
);
System.out.println(user);
}
}
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二、Jackson支持3種使用方式:
1、Data Binding：最方便使用.
(1)Full Data Binding：
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private
static
final
String MODEL_BINDING =
"{\"name\":\"name1\",\"type\":1}"
;
public
void
fullDataBinding()
throws
Exception{
ObjectMapper mapper =
new
ObjectMapper();
Model user = mapper.readValue(MODEL_BINDING, Model.
class
);
//readValue到一個實體類中.
System.out.println(user.getName());
System.out.println(user.getType());
}
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Model類：
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private
static
class
Model{
private
String name;
private
int
type;
public
String getName() {
return
name;
}
public
void
setName(String name) {
this
.name = name;
}
public
int
getType() {
return
type;
}
public
void
setType(
int
type) {
this
.type = type;
}
}
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(2)Raw Data Binding：
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/**
Concrete Java types that Jackson will use for simple data binding are:
JSON Type Java Type
object LinkedHashMap<String,Object>
array ArrayList<Object>
string String
number(no fraction) Integer, Long or BigInteger (smallest applicable)
number(fraction) Double(configurable to use BigDecimal)
true|false Boolean
null null
*/
public
void
rawDataBinding()
throws
Exception{
ObjectMapper mapper =
new
ObjectMapper();
HashMap map = mapper.readValue(MODEL_BINDING,HashMap.
class
);
//readValue到一個原始數據類型.
System.out.println(map.get(
"name"
));
System.out.println(map.get(
"type"
));
}
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(3)generic Data Binding：
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private
static
final
String GENERIC_BINDING =
"{\"key1\":{\"name\":\"name2\",\"type\":2},\"key2\":{\"name\":\"name3\",\"type\":3}}"
;
public
void
genericDataBinding()
throws
Exception{
ObjectMapper mapper =
new
ObjectMapper();
HashMap<String,Model> modelMap = mapper.readValue(GENERIC_BINDING,
new
TypeReference<HashMap<String,Model>>(){});
//readValue到一個范型數據中.
Model model = modelMap.get(
"key2"
);
System.out.println(model.getName());
System.out.println(model.getType());
}
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2、Tree Model：最靈活。
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private
static
final
String TREE_MODEL_BINDING =
"{\"treekey1\":\"treevalue1\",\"treekey2\":\"treevalue2\",\"children\":[{\"childkey1\":\"childkey1\"}]}"
;
public
void
treeModelBinding()
throws
Exception{
ObjectMapper mapper =
new
ObjectMapper();
JsonNode rootNode = mapper.readTree(TREE_MODEL_BINDING);
//path與get作用相同,但是當找不到該節點的時候,返回missing node而不是Null.
String treekey2value = rootNode.path(
"treekey2"
).getTextValue();
//
System.out.println(
"treekey2value:"
+ treekey2value);
JsonNode childrenNode = rootNode.path(
"children"
);
String childkey1Value = childrenNode.get(
0
).path(
"childkey1"
).getTextValue();
System.out.println(
"childkey1Value:"
+childkey1Value);
//創建根節點
ObjectNode root = mapper.createObjectNode();
//創建子節點1
ObjectNode node1 = mapper.createObjectNode();
node1.put(
"nodekey1"
,
1
);
node1.put(
"nodekey2"
,
2
);
//綁定子節點1
root.put(
"child"
,node1);
//數組節點
ArrayNode arrayNode = mapper.createArrayNode();
arrayNode.add(node1);
arrayNode.add(
1
);
//綁定數組節點
root.put(
"arraynode"
, arrayNode);
//JSON讀到樹節點
JsonNode valueToTreeNode = mapper.valueToTree(TREE_MODEL_BINDING);
//綁定JSON節點
root.put(
"valuetotreenode"
,valueToTreeNode);
//JSON綁定到JSON節點對象
JsonNode bindJsonNode = mapper.readValue(GENERIC_BINDING, JsonNode.
class
);
//綁定JSON到JSON節點對象.
//綁定JSON節點
root.put(
"bindJsonNode"
,bindJsonNode);
System.out.println(mapper.writeValueAsString(root));
}
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3、Streaming API：最佳性能。
對於性能要求高的程序,推薦使用流API,否則使用其他方法
不管是創建JsonGenerator還是JsonParser,都是使用JsonFactory。
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package
com.jingshou.jackson;
import
java.io.File;
import
java.io.IOException;
import
com.fasterxml.jackson.core.JsonEncoding;
import
com.fasterxml.jackson.core.JsonFactory;
import
com.fasterxml.jackson.core.JsonGenerator;
import
com.fasterxml.jackson.core.JsonParser;
import
com.fasterxml.jackson.core.JsonToken;
public
class
JacksonTest6 {
public
static
void
main(String[] args)
throws
IOException {
JsonFactory jfactory =
new
JsonFactory();
/*** write to file ***/
JsonGenerator jGenerator = jfactory.createGenerator(
new
File(
"c:\\user.json"
), JsonEncoding.UTF8);
jGenerator.writeStartObject();
// {
jGenerator.writeStringField(
"name"
,
"mkyong"
);
// "name" : "mkyong"
jGenerator.writeNumberField(
"age"
,
29
);
// "age" : 29
jGenerator.writeFieldName(
"messages"
);
// "messages" :
jGenerator.writeStartArray();
// [
jGenerator.writeString(
"msg 1"
);
// "msg 1"
jGenerator.writeString(
"msg 2"
);
// "msg 2"
jGenerator.writeString(
"msg 3"
);
// "msg 3"
jGenerator.writeEndArray();
// ]
jGenerator.writeEndObject();
// }
jGenerator.close();
/*** read from file ***/
JsonParser jParser = jfactory.createParser(
new
File(
"c:\\user.json"
));
// loop until token equal to "}"
while
(jParser.nextToken() != JsonToken.END_OBJECT) {
String fieldname = jParser.getCurrentName();
if
(
"name"
.equals(fieldname)) {
// current token is "name",
// move to next, which is "name"'s value
jParser.nextToken();
System.out.println(jParser.getText());
// display mkyong
}
if
(
"age"
.equals(fieldname)) {
// current token is "age",
// move to next, which is "name"'s value
jParser.nextToken();
System.out.println(jParser.getIntValue());
// display 29
}
if
(
"messages"
.equals(fieldname)) {
jParser.nextToken();
// current token is "[", move next
// messages is array, loop until token equal to "]"
while
(jParser.nextToken() != JsonToken.END_ARRAY) {
// display msg1, msg2, msg3
System.out.println(jParser.getText());
}
}
}
jParser.close();
}
}
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