JSON Plugin

Overview

The JSON plugin provides a "json" result type that serializes actions into JSON. The serialization process is recursive, meaning that the whole object graph, starting on the action class (base class not included) will be serialized (root object can be customized using the "root" attribute). If the interceptor is used, the action will be populated from the JSON content in the request, these are the rules of the interceptor:

1. The "content-type" must be "application/json"
2. The JSON content must be well formed, see json.org for grammar.
3. Action must have a public "setter" method for fields that must be populated.
4. Supported types for population are: Primitives (int,long...String), Date, List, Map, Primitive Arrays, Other class (more on this later), and Array of Other class.
5. Any object in JSON, that is to be populated inside a list, or a map, will be of type Map (mapping from properties to values), any whole number will be of type Long, any decimal number will be of type Double, and any array of type List.

Given this JSON string:

{
   "doubleValue": 10.10,
   "nestedBean": {
      "name": "Mr Bean"
   },
   "list": ["A", 10, 20.20, {
      "firstName": "El Zorro"
   }],
   "array": [10, 20] 
}

The action must have a "setDoubleValue" method, taking either a "float" or a "double" argument (the interceptor will convert the value to the right one). There must be a "setNestedBean" whose argument type can be any class, that has a "setName" method taking as argument an "String". There must be a "setList" method that takes a "List" as argument, that list will contain: "A" (String), 10 (Long), 20.20 (Double), Map ("firstName" -> "El Zorro"). The "setArray" method can take as parameter either a "List", or any numeric array.

Installation

This plugin can be installed by copying the plugin jar into your application's /WEB-INF/lib directory. No other files need to be copied or created.

To use maven, add this to your pom:

<dependencies>
   ...
   <dependency>
       <groupId>com.googlecode</groupId>
       <artifactId>jsonplugin</artifactId>
       <version>0.26</version>
   </dependency>
   ...
</dependencies>
<repository>
    <id>Maven Plugin Repository</id>
    <url>http://struts2plugin-maven-repo.googlecode.com/svn/trunk/</url>
    <snapshots>
      <enabled>false</enabled>
    </snapshots>
    <releases>
      <enabled>true</enabled>
    </releases>
</repository>

Customizing Serialization and Deserialization

Use the JSON annotation to customize the serialization/deserialization process. Available JSON annotation fields:

Excluding properties

A comma-delimited list of regular expressions can be passed to the JSON Result and Interceptor, properties matching any of these regular expressions will be ignored on the serialization process:

<!-- Result fragment -->
<result type="json">
  <param name="excludeProperties">
    login.password,
    studentList.*\.sin
  </param>
</result>

<!-- Interceptor fragment -->
<interceptor-ref name="json">
  <param name="enableSMD">true</param>
  <param name="excludeProperties">
    login.password,
    studentList.*\.sin
  </param>
</interceptor-ref>

Including properties

A comma-delimited list of regular expressions can be passed to the JSON Result to restrict which properties will be serialized. ONLY properties matching any of these regular expressions will be included in the serialized output.

<!-- Result fragment -->
<result type="json">
  <param name="includeProperties">
    ^entries\[\d+\]\.clientNumber,
    ^entries\[\d+\]\.scheduleNumber,
    ^entries\[\d+\]\.createUserId
  </param>
</result>

Root Object

Use the "root" attribute(OGNL expression) to specify the root object to be serialized.

<result type="json">
  <param name="root">
    person.job
  </param>
</result>

The "root" attribute(OGNL expression) can also be used on the interceptor to specify the object that must be populated, make sure this object is not null.

<interceptor-ref name="json">
  <param name="root">bean1.bean2</param>
</interceptor-ref>

Wrap with Comments

If the "wrapWithComments" (false by default) attribute is set to true, the generated JSON is wrapped with comments like:

/* {
   "doubleVal": 10.10,
   "nestedBean": {
      "name": "Mr Bean"
   },
   "list": ["A", 10, 20.20, {
      "firstName": "El Zorro"
   }],
   "array": [10, 20] 
} */

This can be used to avoid potential Javascript Hijacks. To strip those comments use:

var responseObject = eval("("+data.substring(data.indexOf("\/\*")+2, data.lastIndexOf("\*\/"))+")");

Base Classes

By default properties defined on base classes of the "root" object won't be serialized, to serialize properties in all base classes (up to Object) set "ignoreHierarchy" to false in the JSON result:

<result type="json">
  <param name="ignoreHierarchy">false</param>
</result>

Enumerations

By default, an Enum is serialized as a name=value pair where value = name().

  public enum AnEnum {
     ValueA,
     ValueB
  }

  JSON:  "myEnum":"ValueA"

Use the "enumAsBean" result parameter to serialize Enum's as a bean with a special property _name with value name(). All properties of the enum are also serialized.

  public enum AnEnum {
     ValueA("A"),
     ValueB("B");

     private String val;
     
     public AnEnum(val) {
        this.val = val;
     }
     public getVal() {
        return val;
     }
   }

  JSON:  myEnum: { "_name": "ValueA", "val": "A" }

Enable this parameter through struts.xml:

<result type="json">
  <param name="enumAsBean">true</param>
</result>

Compressing the output.

Set the enableGZIP attribute to true to gzip the generated json response. The request must include "gzip" in the "Accept-Encoding" header for this to work.

<result type="json">
  <param name="enableGZIP">true</param>
</result>

Example

Setup Action

This simple action has some fields:

Example:

import java.util.HashMap;
import java.util.Map;

import com.opensymphony.xwork2.Action;

public class JSONExample {
    private String field1 = "str";
    private int[] ints = {10, 20};
    private Map map = new HashMap();
    private String customName = "custom";

    //'transient' fields are not serialized
    private transient String field2;

    //fields without getter method are not serialized
    private String field3;

    public String execute() {
        map.put("John", "Galt");
        return Action.SUCCESS;
    }

    public String getField1() {
        return field1;
    }

    public void setField1(String field1) {
        this.field1 = field1;
    }

    public int[] getInts() {
        return ints;
    }

    public void setInts(int[] ints) {
        this.ints = ints;
    }

    public Map getMap() {
        return map;
    }

    public void setMap(Map map) {
        this.map = map;
    }

    @JSON(name="newName")
    public String getCustomName() {
        return this.customName;
    }
}

Write the mapping for the action

1. Add the map inside a package that extends "json-default"
2. Add a result of type "json"

Example:

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE struts PUBLIC
    "-//Apache Software Foundation//DTD Struts Configuration 2.0//EN"
    "http://struts.apache.org/dtds/struts-2.0.dtd">

<struts>

  <package name="example"  extends="json-default">
     <action name="JSONExample" class="example.JSONExample">
        <result type="json"/>
     </action>
  </package>

</struts>

JSON example output

{  
   "field1" : "str", 
   "ints": [10, 20],
   "map": {
       "John":"Galt"
   },
   "newName": "custom"
}

JSON RPC

The json plugin can be used to execute action methods from javascript and return the output. This feature was developed with Dojo in mind, so it uses Simple Method Definition to advertise the remote service. Let's work it out with an example(useless as most examples).

First write the action:

package smd;

import com.googlecode.jsonplugin.annotations.SMDMethod;
import com.opensymphony.xwork2.Action;

public class SMDAction {
    public String smd() {
        return Action.SUCCESS;
    }
    
    @SMDMethod
    public Bean doSomething(Bean bean, int quantity) {
        bean.setPrice(quantity * 10);
        return bean;
    }
}

Methods that will be called remotely must be annotated with the SMDMethod annotation, for security reasons. The method will take a bean object, modify its price and return it. The action can be annotated with the SMD annotation to customize the generated SMD (more on that soon), and parameters can be annotated with SMDMethodParameter. As you can see, we have a "dummy", smd method. This method will be used to generate the Simple Method Definition (a definition of all the services provided by this class), using the "json" result.

The bean class:

package smd;

public class Bean {
    private String type;
    private int price;
    
    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public int getPrice() {
        return price;
    }

    public void setPrice(int price) {
        this.price = price;
    }

}

The mapping:

<package name="RPC" namespace="/nodecorate" extends="json-default">
    <action name="SMDAction" class="smd.SMDAction" method="smd">
        <interceptor-ref name="json">
            <param name="enableSMD">true</param>
        </interceptor-ref>
        <result type="json">
             <param name="enableSMD">true</param>
        </result>
    </action>
</package>

Nothing special here, except that both the interceptor and the result must be applied to the action, and "enableSMD" must be enabled for both.

Now the javascript code:

<s:url id="smdUrl" namespace="/nodecorate" action="SMDAction" />
<script type="text/javascript">
    //load dojo RPC
    dojo.require("dojo.rpc.*");
    
    //create service object(proxy) using SMD (generated by the json result)
    var service = new dojo.rpc.JsonService("${smdUrl}");
    
    //function called when remote method returns
    var callback = function(bean) {
        alert("Price for " + bean.type + " is " + bean.price);
    };
    
    //parameter
    var bean = {type: "Mocca"};
    
    //execute remote method
    var defered = service.doSomething(bean, 5);

    //attach callback to defered object
    defered.addCallback(callback);
</script>

Dojo's JsonService will make a request to the action to load the SMD, which will return a JSON object with the definition of the available remote methods, using that information Dojo creates a "proxy" for those methods. Because of the asynchronous nature of the request, when the method is executed, a deferred object is returned, to which a callback function can be attached. The callback function will receive as a parameter the object returned from your action. That's it.

Proxied objects

(V0.20) As annotations are not inherited in Java, some user might experience problems while trying to serialize objects that are proxied. eg. when you have attached AOP interceptors to your action.

In this situation, the plugin will not detect the annotations on methods in your action.

To overcome this, set the "ignoreInterfaces" result parameter to false (true by default) to request that the plugin inspects all interfaces and superclasses of the action for annotations on the action's methods.

NOTE: This parameter should only be set to false if your action could be a proxy as there is a performance cost caused by recursion through the interfaces.

<action name="contact" class="package.ContactAction" method="smd">
   <interceptor-ref name="json">
      <param name="enableSMD">true</param>
      <param name="ignoreInterfaces">false</param>
   </interceptor-ref>
   <result type="json">
      <param name="enableSMD">true</param>
      <param name="ignoreInterfaces">false</param>
   </result>
   <interceptor-ref name="default"/>
</action>

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