How to create multi module project with maven?

It is more than three years, I have started to use maven and it is a great tool, I have ever used. The support of repository management and the features available to create multi module project are excellent. With this tutorial, I am going to show you, how to create a multi module project with maven. I am using eclipse as IDE and 'm2eclipse' plugin for eclipse.

The following picture shows the project structure in eclipse.

My project is 'shims' and it has two modules as 'shims-core' and 'shims-web'. One module contains all business logic classes and data access objects which is bundled as jar file. And the other module is the web module which contains controller related classes (Struts2 action or Servlets or Spring controller classes) and view related stuff.

The tutorial summary is as follows.

• Installing maven
• Configure m2eclipse, maven with eclipse
• Creating parent project
• Creating child jar module
• Creating child war module
• Building multi module project

Installing maven

Download the Apache maven distribution and extract it some where in your computer. You can download the apache maven from here. I have extracted the distribution into the following location in my computer.

/home/vinesh/apache-maven-3.0.4

Next, You have to set 'MAVEN_HOME' environmental variable and add the path of maven's bin folder to classpath. In Ubuntu, you can set the variable as follows. You have to add the following two lines into bashrc file. First, run the following command to open 'bashrc' file.

$ vi ~/.bashrc

And add the following two lines.

export MAVEN_HOME=/home/vinesh/apache-maven-3.0.4
export PATH=$MAVEN_HOME/bin:$PATH

In windows, setting an environment variable is not that much difficult. So I am not going to explain that in this tutorial.

After editing 'bashrc' file, just open a command window and run the following command.

$ mvn -version

If you get the output similar to following, you are successfully configured apache maven.

Apache Maven 3.0.4 (r1232337; 2012-01-17 14:14:56+0530)
Maven home: /home/semika/apache-maven-3.0.4
Java version: 1.6.0_30, vendor: Sun Microsystems Inc.
Java home: /home/semika/java/jdk1.6.0_30/jre
Default locale: en_US, platform encoding: UTF-8
OS name: "linux", version: "2.6.38-16-generic", arch: "amd64", family: "unix"

You are done with the first step and let's move to next step.  

Configure m2eclipse, maven with eclipse 

You have to install m2eclipse plugin first. You can install m2eclipse plugin via eclipse it self. I am using eclipse 'Indigo' version and there may be slight differences with other version of eclipse. 

This is the update URL for m2eclipse.

http://download.eclipse.org/technology/m2e/releases

Go to Help --> Install New Software. You will get the following window.

You can add the above update URL and continue the wizard. Normally this will take some time and after finishing the installation, you have to restart the eclipse. Go to Window -->Preferences. You should see that 'Maven' category has added to preferences categories.

We are not going to use embedded maven coming with the plugin as I have highlighted in the above image. Instead, we are going to configure our maven installation with eclipse. 

Click the 'Add' button and locate your maven installation directory. For further clarification see the image bellow. 

You can see now, the plugin has been configured with our maven installation. Now, you are ready to create projects using m2eclipse. Let's start to create our multi module project.

Eclipse provides great facilities to create multi module projects and relationships among the modules. In this tutorial, I am going to create a maven multi module projects from the scratch.




 

Creating maven parent project

Step01: Go to File --> New --> Project.

You will get 'New Project' window. Under 'Maven' category, select 'Maven Project'. Click 'Next' button. You will get 'New Maven Project' window. See the bellow image.

Step 02: We are going to create simple maven project. As I have pointed out from right side image, make sure to check 'Create a simple project' checkbox. Click on 'Next' button. Then you will get the following window.

I feel, it is better to give little explanation about the four fields highlighted with red color box in the right side image.

Group Id: This is the root packaging structure of your project. Make sure to use the same root package structure for every module in the project. 

Artifact Id : This is similar to your project module name. In right side image, I have given 'shims' as the artifact id. That is the parent project of my all project modules.

Packaging: This is very important when creating multi module project with maven. There are several options which you can select as the packaging. If you want to create a parent project, we are doing like now, you have to select 'pom' as the packaging. To create Java module, select 'jar' and for web module, select 'war'. I will explain later on this tutorial, how to create a 'jar' and 'war' module. You know that 'shims-core' is a jar module which contains all the business logic and data access objects and 'shims-web' is a web module containing all controller  and view related stuff.

Click the 'Finish' button and see, maven is creating the parent project for our multi module project. Let's little examine this.

Open the 'pom.xml' file. It should be similar to the following.

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.semika</groupId>
  <artifactId>shims</artifactId>
  <version>1.0-SNAPSHOT</version>
  <packaging>pom</packaging>
</project>

Let's create our first child module.  

Creating maven child jar module

We want to keep our all business logic and DAO classes as a separate module which can be bundled to as a 'jar' file. We can create a separate project module for this purpose. In my sample application, it has 'shims-core' module. This is a child module of our parent project which we already created. 

To create child jar module.

Step 01: Right click on the parent project, Go to New-->Project. Select the 'Maven Module' from the 'Maven' category. As you can remember, we selected 'Maven Project' when we create our parent project before. Since we are creating a project module now, we should select 'Maven Module'. 

Click the 'Next' button. You will get 'New Maven Module' window as follows.

Make sure to check 'Create a simple project' check box and give a module name. You can see that 'shims' which is our parent project has automatically selected as the 'Parent Project'. We are in a correct way. Click the 'Next' button. You will get familiar window as follows.

As You can see in the right side image, 'Group Id', 'Artifact Id' and 'Version' have already been set by default. We don't need to change any of those three field values. Keep those values as it is. Since we are creating 'jar' module, we should select 'jar' as the packaging. Click the 'Finish' button and see maven is creating our first child module.

Now refresh the parent project and notice that maven has created a new project 'shims-core' inside the 'shims'. The 'shims-core' represents a another maven project module which has separate pom.xml file. Also 'shims-core' represents as the new project within eclipse.

Just have a look on following image.

Now, If you see parent project's pom.xml file, you can see that 'shims-core' has listed under modules which says that 'shims-core' is a child module of 'shims' parent project.

  <modules>
   <module>shims-core</module>
  </modules>

Also, in the child project's pom.xml file, it has a reference to parent project. Further, in child module's pom.xml file, 'groupId' and 'version' have not been defined. Those are inherited from parents project. This is good practice when you are creating multi module project with maven.

  <parent>
    <artifactId>shims</artifactId>
    <groupId>com.semika</groupId>
    <version>1.0-SNAPSHOT</version>
    <relativePath>..</relativePath>
  </parent>

Creating child war module

Next, we will create our web module, 'shims-web' which contains our controller classes and other view related stuff. Creating war module with m2eclipse is little different rather creating jar module. We have to use one of the maven pre-defined archetype  for web module creation. Don't worry, I will give you step by step explanation about creating maven war module. 

Step 01: Right click on parent project, Go to New-->Project and select 'Maven Module', Click 'Next' button.

In this case, we are not going to create a simple project. So don't check on 'Create simple project' check box. As you can see from the right side image, parent project has automatically selected as 'shims' which is our parent project. You have to specify the module name as 'shims-web'. and click 'Next' button.




Step 02: Click the 'Next' button from the above image. We are going to use 'maven-archetype-webapp' artifact to create our war module. See the bellow image.

Step 03: Click 'Next'  button from above window and you will get the following window.

You only need to change the 'Package' from right side window. Click the 'Finish' button and let the maven create our web module. After creating the new web module, refresh the parent project and notice that it has a new module as 'shims-web'. So now we have a multi module project with tow modules. Open pom.xml file from the web module and make sure to remove 'groupId' and 'version'. Also see the parent project's pom.xml file and notice that 'shims-web' has been added to it as a new module.

We hope to deploy our application as a war bundle into the server. If you want to change you war file name, you can change 'filaName' attribute of pom.xml file of 'shims-web' module.

  <build>
    <finalName>shims-web</finalName>
  </build>

This will create 'shims-web.war' file under 'target' directory of 'shims-web' module. Next, we need to define dependencies between 'shims-core' and 'shims-web' module. This will force the maven reactor to build 'shims-core' module before building 'shims-web' module when we run maven build from the parent project. Because, we want to bundle our business logic and DAO related classes into a separate jar module and give it to web module as a separate library along with other libraries, core module should be build before web module.

Place the following contents inside the 'dependencies' section of pom.xml file of 'shims-web' module.

    <dependency>
        <groupId>${project.groupId}</groupId>
        <artifactId>shims-core</artifactId>
        <version>${project.version}</version>
    </dependency>

${project.groupId} and ${project.version} are two global references of maven which provides 'groupId' and 'version' across the whole modules of the multi module project. We can directly use it.

Building multi module project

There are lots of advantages of creating multi module project. We can build the entire project at once and also we can build each module separately. Developers can separately work on different modules at the same time without depending on others. Other thing is, we have clear separation which enhance maintainability.

We have to create maven targets to build our project. Right click on parent project. Go to Run As --> Maven Build. You will get the following window only for the very first time. There you can create a new target to build your project. 

The right side window allow you to create maven target to build your project. You have to give a name for your build target and browse the required project module directory. Since I am going to create this maven target to build the entire project, I have located my parent directory. Put 'clean install' into the Goals field.

Since, we are going to build our project very first time, we should allow maven to download the required dependencies from their global repository. Make sure 'offline' check box 'unchecked'. After first success build, you can update this target to run it in offline mode by selecting 'offline' check box.

Make sure 'Maven Runtime' has been set to our maven installation directory. Otherwise you will get build errors.

OK!. That's it. You are ready to build your first maven multi module project. Good luck!

Click on 'Apply' and then 'Run' button. Maven will start the building project. See carefully eclipse console. It says full story. You should get 'SUCCESS' message at the end of the build. Similar output as follows.

-------------------------------------
[INFO] Installing /home/vinesh/workspace-maaven/shims/shims-web/pom.xml to /home/vinesh/.m2/repository/com/semika/shims-web/1.0-SNAPSHOT/shims-web-1.0-SNAPSHOT.pom
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO] [INFO] shims ............................................. SUCCESS [1.093s]
[INFO] shims-core ........................................ SUCCESS [2.005s]
[INFO] shims-web Maven Webapp ............................ SUCCESS [1.027s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 4.354s
[INFO] Finished at: Sat Nov 03 22:37:27 IST 2012
[INFO] Final Memory: 6M/103M
[INFO] ------------------------------------------------------------------------

Let's deploy our application. I am using Apache tomcat7 to deploy my application. If you see inside the target directory of 'shims-web' module after the successful build, you can see,maven has create 'shims-web.war' file. Copy that file into tomcat's webapps folder and start the tomcat.

Point your browser the following URL.

http://localhost:8080/shims-web/

You should get hello world page. 

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Full stack trace for "SEVERE: Error listenerStart"

One of the very painful error, I have received when starting the tomcat 'SEVERE: Error listenerStart'. I can not even understand why tomcat is giving errors in such a poor manner by wasting lots of our time. This is server error, but it does not give enough information to get rid of that. 

As it says, this may be some configuration problems which failed creation of some listener. For example, it may be some problem with your spring configuration which prevents creating ContextLoaderListener.

Sometime, this may be a problem of misconfiguration of a filter. You may have configured a filter in web.xml, but the relevant filter class is not found. 

Most of you will happy, if we can get the full stack trace of above exception.You can create 'logging.properties' file in your project source so that, it will be in 'classes' folder  of your deployment archive. You can place it within 'src' folder or within 'resources' folder in your source based on your project type. The project may be tomcat project or maven project or dynamic web project. 

After creating that file, place the following contents within it.

org.apache.catalina.core.ContainerBase.[Catalina].level = INFO
org.apache.catalina.core.ContainerBase.[Catalina].handlers = java.util.logging.ConsoleHandler

Now try to start the tomcat and you will see full stack trace of above poor indication.

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How to place the Facebook "Like Box" to your web site or Blog?

One of the very useful social plugin provided by the Facebook is 'Like Box'. If you have placed Facebook like box in your website or blog, then your visitors can make 'likes' to it. This will not be a partial tutorial. I don't want to leave any thing unclear about this. At the end of this tutorial, you must be able to add Facebook like box into your website or blog.

Just look at the right side bar of my blog. You can see Facebook like box there. Unfortunately by this time, my wife and my self has liked to my Blog. I hope, you will also like my blog after reading this tutorial.

OK, let's move to real topic!

Go to this URL,

http://developers.facebook.com/docs/plugins/

There you can see all the plugins available with Facebook. Just scroll down and you can see 'Like Box' plugin for you. Click on that link and you will get the following form.

I guess, most of the fields of the right side form are clear for you. But the first field which is 'Facebook Page URL', confuses us. Some people think, this should be your blog or website URL and some are thinking, this should be a Facebook's application URL. So they create new Facebook application and try to get the Facebook like box. Actually as it says, it should be the Facebook page URL. You have to create a Facebook page so that your website or blog will get likes via that page. Facebook 'Like Box' is the plugin which connects Facebook page with our website or blog. We don't actually want visitors to visit Facebook page, but we want, visitors should visit our website or blog. The ultimate target is, we want to allow our visitors of blog or website to give their impression on it by giving a 'likes'. As the first step, let,s create Facebook page first. Login to your Facebook account and click on 'Like Pages' link on left side bar under PAGES category of Facebook home page. You will get the following page.

You can see 'Create a Page' green color button on right top area of the page. Click on that and you will get the following page.

You can see several categories on this page. Select the one appropriate for you and provide the required details. It is important to provide unique name for your page, because visitors will see that on website or blog. I selected 'Company, Organization or Institution' and my page name is 'Lasanthiblog'. You can see it from bellow image.

After providing a good name for your page, proceed by clicking on 'Get Started' button. You will get the following page which is very important.

As you can see from this image, we have to complete four steps to create our Facebook page. As the first step upload a picture for your Facebook page profile and click 'Save Photo' button. You will be directed to second step. See the picture bellow.

You have to provide some description about your page and URL of your website or blog. I have provided my blog's URL as you can see. Go to next step by clicking on 'Save Info' button. Next page will be displayed as follows.

This step is very important. You will get the public URL of your page from here. You can select proper and meaningful name for your page URL. As you can see, I have given 'lasanthiblog' as page address. Click on 'Set Address' button and you can skip the next step if you don't want to enable adds.

After completing the 4th step, you will be redirected to your page. My page is shown bellow. Actually this is not my real page, I created this page to demonstrate, how to create a Facebook page for you.

Now the hard part is done and you are ready to get Facebook like plugin for your website or blog. Now we have Facebook page RUL.

http://www.facebook.com/lasanthiblog

Now the time to get code for our Facebook like box plugin. Again go to the Facebook plugin page.

http://developers.facebook.com/docs/plugins/

Select 'Like Box' and fill the form. After filling the form click on 'Get Code' button. You can get the code in four different ways. But I suggest you to get 'IFRAME' code.

Now you place this code in your website or blog, where you want to place Facebook like plugin. I guess, you can manage that by your selves. I hope this will be helpful for you and if you are still having problem of getting Facebook 'Like Box', please mail to me. I will help you. OK!, go ahead.

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[ERROR] error: error reading *.jar; invalid LOC header (bad signature) - Maven build error

Some time you will get the following error when building a project with maven. Most of the time, this happens after checking out new maven project and try to build the project with an existing repository. Some existing jar files can not be read when building the new project. This may happen because, you may already have some projects sharing the same repository.

The following shows a sample error.

[ERROR] error: error reading /home/vinesh/.m2/repository/org/codehaus/woodstox/stax2-api/3.1.1/stax2-api-3.1.1.jar; invalid LOC header (bad signature)

As a solution for this, you can remove the existing jar file and build the project again in online mode. You can use the following command.

mvn clean install -Dmaven.test.skip=true

Now, carefully look into the console.You can see, it downloads the removed jar file from the maven repository.

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Make your all AJAX request through a single gateway.

AJAX has become a most popular and widely used technology in modern web application. In some web application, around 95% of the server requests are AJAX requests. When you are developing your application, it is always a good practice finding more generic and reusable ways of implementing things. Sometime, we may call as inventing new patterns. 

This tutorial explains a pattern of making your all AJAX requests through a single gateway. I am not explaining a technology feature, but this is programing technique. 

What are the advantages?

• Suppose you want to pass some common set of parameters with every AJAX request. This can be easily achieved by this technique. 
• If your application has an exception flow mechanism which brings exception messages happened in your DAO layer or service layer or even in the controller layer into the client view, those can be easily handled in central point with this technique. 
• If you want to show each operations status to the user, that can also be handled from a central point.

There may be more advantages.I will give you a sample implementation with jQuery. But this can be implemented with any Javascript library as you wish.

If you use jQuery, the following is the typical Javascript method using for AJAX request.

$.ajax({
        type: 'POST',
        url: CONTEXT_ROOT + "/changePassword.htm",
        data: $.blimp.serializeForm("frmChangePwd"),
        success: function (response) {
            //handle success
        },
        error: function () {
            //handle errors
        },
        dataType: 'json'
});

For my application, I have written a common central method to make every AJAX requests. This method works as a gateway which all AJAX requests are passing through this method. Any common functionality that we need to add before making the request as well as after making the request, can easily be added to this common method. The following code shows the common gateway method.

/**
 * @author Semika siriwardana
 * 
 */
(function($) {
     $.myObj = {
  /**
   * Central AJAX caller gateway.
   */
 ajax: function(objConfig) {
   
     $.ajax({
     type: objConfig.type, //mandatory field
     url: objConfig.url, //mandatory
     headers: (objConfig.headers != undefined)?objConfig.headers:{}, 
     async: (objConfig.async != undefined)?objConfig.async:true,
     data : (objConfig.data != undefined)?objConfig.data:{},
     dataType : (objConfig.dataType != undefined)?objConfig.dataType:'json',
     cache: (objConfig.cache != undefined)?objConfig.cache:true,
     contentType: (objConfig.contentType != undefined)?objConfig.contentType:'application/x-www-form-urlencoded',
       
           success: function(data) {
               //Handle success status and do more
               objConfig.success(data); //Invoke success callback method.   
           },
             
           error: function(data, textStatus, jqXHR) {
                       //Handle error status and do more
                objConfig.error(data, textStatus, jqXHR)   
           }
     });
 }
     }
}(jQuery));

As I mentioned before, this is a programing technique or a pattern. You know that '$.ajax()' is the method provided by jQuery for AJAX request. As you see above, I have written a wrapper method as '$.myObj.ajax(objConfig)'. Now onwords, when you want to make any AJAX requests, you are not directly invoking original '$.ajax()' method, but you invoke the wrapper method '$.myObj.ajax(objConfig)'. 

Next, you will have a problem. What is this 'objConfig' parameter passing into '$.myObj.ajax()' method?. You will get the answer soon.
To make your AJAX request through our gateway method, you just need to change '$.ajax' to '$.myObj.ajax' from the above code.

myObj.ajax({
       type: 'POST',
       url: CONTEXT_ROOT + "/changePassword.htm",
       data: $.blimp.serializeForm("frmChangePwd"),
       success: function (response) {
            //handle success
       },
       error: function () {
            //handle errors
       },
       dataType: 'json'
});

If you use '$.myObj.ajax' for your all AJAX requests, you can easily handle some common behaviours with AJAX invocation.

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