使用Jackson在Java中处理JSON
前一段工作中总会遇到使用Java处理JSON的情况，大部分都使用的是开源工具Jackson实现的。
因此总结一下发上来，希望对看到的人有所帮助。
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一、入门
Jackson中有个ObjectMapper类很是实用，用于Java对象与JSON的互换。
1、Java对象转换为JSON：
User user=new User(); //Java Object
ObjectMapper mapper = new ObjectMapper();
mapper.writeValueAsString(user); //返回字符串
//输出格式化后的字符串(有性能损耗)
mapper.defaultPrettyPrintingWriter().writeValueAsString(user);
mapper.writeValue(new File("c:\\user.json"), user); //指定文件写入
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//设置序列化配置(全局),设置序列化时不输出空值.
sharedMapper.getSerializationConfig().setSerializationInclusion(Inclusion.NON_NULL);
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2、JSON反序列化为Java对象：
ObjectMapper mapper = new ObjectMapper();
//解析器支持解析单引号
mapper.configure(Feature.ALLOW_SINGLE_QUOTES,true);
//解析器支持解析结束符
mapper.configure(Feature.ALLOW_UNQUOTED_CONTROL_CHARS,true);
HashMap jsonMap = mapper.readValue(json,HashMap.class); //转换为HashMap对象
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二、Jackson支持3种使用方式:
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到一个实体类中.System.out.println(user.getName());System.out.println(user.getType());}
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;}}
/**Concrete Java types that Jackson will use for simple data binding are:JSON Type Java TypeobjectLinkedHashMap<String,Object>arrayArrayList<Object>stringStringnumber(no fraction)Integer, Long or BigInteger (smallest applicable)number(fraction)Double(configurable to use BigDecimal)true|falseBooleannullnull*/public void rawDataBinding() throws Exception{ObjectMapper mapper = new ObjectMapper();HashMap map = mapper.readValue(MODEL_BINDING,HashMap.class);//readValue到一个原始数据类型.System.out.println(map.get("name"));System.out.println(map.get("type"));}
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到一个范型数据中.Model model = modelMap.get("key2");System.out.println(model.getName());System.out.println(model.getType());}
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与get作用相同,但是当找不到该节点的时候,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);//创建根节点ObjectNode root = mapper.createObjectNode();//创建子节点1ObjectNode node1 = mapper.createObjectNode();node1.put("nodekey1",1);node1.put("nodekey2",2);//绑定子节点1root.put("child",node1);//数组节点ArrayNode arrayNode = mapper.createArrayNode();arrayNode.add(node1);arrayNode.add(1);//绑定数组节点root.put("arraynode", arrayNode);//JSON读到树节点JsonNode valueToTreeNode = mapper.valueToTree(TREE_MODEL_BINDING);//绑定JSON节点root.put("valuetotreenode",valueToTreeNode);//JSON绑定到JSON节点对象JsonNode bindJsonNode = mapper.readValue(GENERIC_BINDING, JsonNode.class);//绑定JSON到JSON节点对象.//绑定JSON节点root.put("bindJsonNode",bindJsonNode);System.out.println(mapper.writeValueAsString(root));}
3、Streaming API：最佳性能。
见官方文档例子。
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参考资料:
1、http://wiki.fasterxml.com/JacksonInFiveMinutes?Jackson官方教程示例
2、http://wiki.fasterxml.com/JacksonJavaDocs?Jackson在线API文档
3、http://hjg1988.iteye.com/blog/561368?JSON工具性能比较:json-lib和jackson进行Java对象到json字符串序列化。
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