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Convention Plugin

 
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Hello world

Now that the Convention plugin has been added to your application, let's start with a very simple example. This example will use an actionless result that isidentified鉴定by the URL. By default, the Convention plugin assumes承担 that all of the results are stored inWEB-INF/content. This can be changed by setting the propertystruts.convention.result.pathin the Struts properties file to the new location. Don't worry about trailing slashes斜杠, the Convention plugin handles this for you. Here is our hello world JSP:

<html>
<body>
Hello world!
</body>
</html>

If you start Tomcat (or whichever J2EE container容器 you are using) and type inhttp://localhost:8080/hello-worldinto your browser you should get this result:

WEB-INF/content/hello-world.jsp 
Hello world!

This illustrates说明 that the Convention plugin will find results even when no actionexists存在 and it is all based on the URL passed to Struts.

Code behind hello world

Let's expand on详述 this example and add a code behind class. In order to do this we need to ensure保证 that the Convention plugin is able to find our action classes. By default, the Convention plugin will find all action classes that implementcom.opensymphony.xwork2.Actionor whose name ends with the wordActionin specific packages.

These packages are located处于 by the Convention plugin using a search methodology. First the Convention plugin finds packages namedstruts,struts2,actionoractions. Any packages that match those names are considered the root packages for the Convention plugin. Next, the plugin looks at all of the classes in those packages as well as sub-packages and determines if the classes implementcom.opensymphony.xwork2.Actionor if their name ends withAction(i.e. FooAction). Here's an example of a few classes that the Convention plugin will find:

Classes 
com.example.actions.MainAction
com.example.actions.products.Display (implements com.opensymphony.xwork2.Action)
com.example.struts.company.details.ShowCompanyDetailsAction

Each of the action classes that the plugin finds will be configured to respond响应to specific URLs. The URL is based on the package name that the class is defined in and the class name itself. First the plugin determines the namespace of the URL using the package names between the root package and the package the class is defined in. For our examples above, the namespaces would be:

Namespaces 
com.example.actions.MainAction -> /
com.example.actions.products.Display -> /products
com.example.struts.company.details.ShowCompanyDetailsAction -> /company/details

Next, the plugin determines the URL of the resource using the class name. It first removes the wordActionfrom the end of the class name and then converts camel case names to dashes. In our example the full URLs would be:

Full URLs 
com.example.actions.MainAction -> /main
com.example.actions.products.Display -> /products/display
com.example.struts.company.details.ShowCompanyDetailsAction -> /company/details/show-company-details

You can tell the Convention plugin to ignore certain packages using the propertystruts.convention.exclude.packages. You can also tell the plugin to use different strings to locate root packages using the propertystruts.convention.package.locators. Finally, you can tell the plugin to search specific root packages using the propertystruts.convention.action.packages.

Here is our code behind action class:

com.example.actions.HelloWorld 
package com.example.actions;

import com.opensymphony.xwork2.ActionSupport;

public class HelloWorld extends ActionSupport {
  private String message;

  public String getMessage() {
    return message;
  }

  public String execute() {
    message = "Hello World!";
    return SUCCESS;
  }
}

If you compile this class and place it into your application in the WEB-INF/classes, the Convention plugin will find the class and map the URL/hello-worldto it. Next, we need to update our JSP to print out the message we setup in the action class. Here is the new JSP:

WEB-INF/content/hello-world.jsp 
<html>
<body>
The message is ${message}
</body>
</html>

If start up the application server and open uphttp://localhost:8080/hello-worldin our browser, we should get this result:

Result 
The message is Hello World!

Results and result codes

The Convention Plugin will pre-configure all of you action classes when Struts is started. By default, this configuration will also contain results for any JSPs that it can find within the application. The JSPs have an additional feature that allows different JSPs to be used based on the result code of the action. Since action methods return Strings and these Strings are traditionally used to locate results for the action, the Convention plugin allows you to define different results based on the result code.

Building on our example from above, let's say we want to provide a different result if the result code from our action is the Stringzerorather thansuccess. First, we update the action class to return different result codes:

com.example.actions.HelloWorld 
package com.example.actions;

import com.opensymphony.xwork2.ActionSupport;

public class HelloWorld extends ActionSupport {
  private String message;

  public String getMessage() {
    return message;
  }

  public String execute() {
    if (System.currentTimeMillis() % 2 == 0) {
      message = "It's 0";
      return "zero";
    }

    message = "It's 1";
    return SUCCESS;
  }
}

Next, we add a new JSP to the application namedWEB-INF/content/hello-world-zero.jsp. Notice that the first part of the file name is the same as the URL of the action and the last part of the name is the result code. This is the convention that the plugin uses to determine which results to render. Here is our new JSP:

WEB-INF/content/hello-world.jsp 
<html>
<body>
The error message is ${message}
</body>
</html>

Now, if you compile the action and restart the application, based on the current time, you'll either see the result fromWEB-INF/content/hello-world.jsporWEB-INF/content/hello-world-zero.jsp.

The result type is based on the extension of the file. The supported extensions are: jsp,ftl,vm,html,html. Examples of Action and Result to Template mapping:

URL Result File that could match Result Type
/hello success /WEB-INF/content/hello.jsp Dispatcher
/hello success /WEB-INF/content/hello-success.htm Dispatcher
/hello success /WEB-INF/content/hello.ftl FreeMarker
/hello-world input /WEB-INF/content/hello-world-input.vm Velocity
/test1/test2/hello error /WEB-INF/content/test/test2/hello-error.html Dispatcher

Chaining

If one action returns the name of another action in the same package, they will be chained together, if the first action doesn't have any result defined for that code. In the following example:

com.example.actions.HelloWorld 
package com.example.actions;

import com.opensymphony.xwork2.Action;
import com.opensymphony.xwork2.ActionSupport;

public class HelloAction extends ActionSupport {
    @Action("foo")
    public String foo() {
        return "bar";
    }

    @Action("foo-bar")
    public String bar() {
        return SUCCESS;
    }
}

The "foo" action will be executed, because no result is found, the Convention plugin tries to find an action named "foo-bar" on the same package where "foo" is defined. If such an action is found, it will be invoked using the "chain" result.

XWork packages

Actions are placed on a custom XWork package which prevents conflicts. The name of this package is based on the Java package the action is defined in, the namespace part of the URL for the action and the parent XWork package for the action. The parent XWork package is determined based on the property namedstruts.convention.default.parent.package(defaults toconvention-default), which is a custom XWork package that extendsstruts-default.

Therefore the naming for XWork packages used by the Convention plugin are in the form:

XWork package naming 
<java-package>#<namespace>#<parent-package>

Using our example from above, the XWork package for our action would be:

XWork package naming 
com.example.actions#/#conventionDefault

Annotation reference

The Convention plugin uses a number of different annotations to override the default conventions that are used to map actions to URLs and locate results. In addition, you can modify the parent XWork package that actions are configured with.

Action annotation

The Convention plugin allows action classes to change the URL that they are mapped to using theActionannotation. This annotation can also be used inside theActionsannotation to allow multiple URLs to map to a single action class. This annotation must be defined on action methods like this:

com.example.actions.HelloWorld 
package com.example.actions;

import com.opensymphony.xwork2.ActionSupport;
import org.apache.struts2.convention.annotation.Action;

public class HelloWorld extends ActionSupport {
  @Action("/different/url")
  public String execute() {
    return SUCCESS;
  }
}

Our action class will now map to the URL/different/urlrather than/hello-world. If no@Result(see next section) is specified, then the namespace of the action will be used as the path to the result, on our last example it would be/WEB-INF/content/different/url.jsp.

A single method within an action class can also map to multiple URLs using theActionsannotation like this:

com.example.actions.HelloWorld 
package com.example.actions;

import com.opensymphony.xwork2.ActionSupport;
import org.apache.struts2.convention.annotation.Action;
import org.apache.struts2.convention.annotation.Actions;

public class HelloWorld extends ActionSupport {
  @Actions({
    @Action("/different/url"),
    @Action("/another/url")
  })
  public String execute() {
    return SUCCESS;
  }
}

Another usage of theActionorActionsannotation is to define multiple action methods within a single action class, each of which respond to a different URL. Here is an example of multiple action methods:

com.example.actions.HelloWorld 
package com.example.actions;

import com.opensymphony.xwork2.ActionSupport;
import org.apache.struts2.convention.annotation.Action;
import org.apache.struts2.convention.annotation.Actions;

public class HelloWorld extends ActionSupport {
  @Action("/different/url")
  public String execute() {
    return SUCCESS;


  }

  @Action("url")
  public String doSomething() {
    return SUCCESS;
  }
}

The previous example defines a second URL that is not fully qualified. This means that the namespace for the URL is determined using the Java package name rather than the Action annotation.

Interceptor and interceptor stacks can be specified using theinterceptorRefsattribute. The following example applies thevalidationinterceptor and thedefaultStackinterceptor stack to the action:

com.example.actions.HelloWorld 
package com.example.actions;

import com.opensymphony.xwork2.ActionSupport;
import org.apache.struts2.convention.annotation.Action;
import org.apache.struts2.convention.annotation.Actions;

public class HelloWorld extends ActionSupport {
  @Action(interceptorRefs={@InterceptorRef("validation"), @InterceptorRef("defaultStack")})
  public String execute() {
    return SUCCESS;
  }

  @Action("url")
  public String doSomething() {
    return SUCCESS;
  }
}

Parameters can be passed to results using theparamsattribute. The value of this attribute is a string array with an even number of elements in the form {"key0", "value0, "key1", "value1" ... "keyN", "valueN"}. For example:

com.example.actions.HelloWorld 
package com.example.actions;

import com.opensymphony.xwork2.ActionSupport;
import org.apache.struts2.convention.annotation.Action;
import org.apache.struts2.convention.annotation.Actions;

public class HelloWorld extends ActionSupport {
  @Action(interceptorRefs=@InterceptorRef(value="validation",params={"programmatic", "false", "declarative", "true}))
  public String execute() {
    return SUCCESS;
  }

  @Action("url")
  public String doSomething() {
    return SUCCESS;
  }
}

If interceptors are not specified, the default stack is applied.

You can specify className parameter which can be especially useful when Spring Framework is used to instantiate actions.

Applying @Action and @Actions at the class level

There are circumstances when this is desired, like when usingDynamic Method Invocation. If anexecutemethod is defined in the class, then it will be used for the action mapping, otherwise the method to be used will be determined when a request is made (by Dynamic Method Invocation for example)

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