jackson序列化key排序

对象在序列化的时候对key进行排序

使用 

JsonPropertyOrder
``` java

@Target({ElementType.ANNOTATION_TYPE, ElementType.TYPE,
    ElementType.METHOD, ElementType.CONSTRUCTOR, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@JacksonAnnotation
public @interface JsonPropertyOrder
{
    /**
     * Order in which properties of annotated object are to be serialized in.
     *  指定key的排序
     */
    public String[] value() default { };

    /**
     * Property that defines what to do regarding ordering of properties
     * not explicitly included in annotation instance. If set to true,
     * they will be alphabetically ordered; if false, order is
     * undefined (default setting)
　　　*  为true的时候，则按字母排序。
     */
    public boolean alphabetic() default false;
}
```

测试
``` java

public class TestJackson {

    public static void main(String[] args) throws IOException {
        TestA a = new TestA();
        a.setA("");
        a.setB("");
        a.setC("");
        a.setD("");
        a.setE("");
        a.setF("");
        a.setG("");
        a.setH("");
        a.setI("");
        a.setJ("");
        a.setK("");
        a.setL("");
        a.setZ("");
        System.out.println(JsonUtil.encode(a));
    }
    @Data
    @JsonPropertyOrder(alphabetic = true)
    public static class TestA{
        private String z= "";
        private String b = "2";
        private String d= "";
        private String t= "";
        private String c= "";
        private String g= "";
        private String k= "";
        private String f= "";
        private String e= "";
        private String a = "1";
        private String h= "";
        private String i= "";
        private String j= "";
        private String l= "";
        private String a1= "";
        private String a2= "";
        private String a3= "";
    }
}

```
结果：
``` json
{"a":"","a1":"","a2":"","a3":"","b":"","c":"","d":"","e":"","f":"","g":"","h":"","i":"","j":"","k":"","l":"","t":"","z":""}
```