Unit Testing JSP Custom Tag Using Spring

In order to unit test your JSP tag, you need to mock up ServletContext and PageContext.  This would be a difficult task without the help from 3rd party library.  Spring 2.5 provides a mock up liberary bundled in spring-test-2.5.x.jar.  This greatly speed up the construction of the test code.

Now I'm going to show an example on testing a custom tag using Spring test library:

Assuming we have a custom tag as below:

public class DataInquiryTag extends TagSupport {

 private String name;  // attribute parameter name
 private String size;  // HTML element size 
 
 @Override
 public int doStartTag() throws JspException {
  String msg = "Hello "+name+"";
  pageContext.getOut().write(msg);
  return super.doStartTag();
 } 
}

We test our tag using JUnit 4.
Before every test invocation, we need to mock ServletContext and PageContext. To do that we setup @Before method as below:

@Before
 public void setup(){
  // mock ServletContext
  mockServletContext = new MockServletContext();
  // mock PageContext
  mockPageContext = new MockPageContext(mockServletContext);  
  tag = new MyTag();
  tag.setPageContext(mockPageContext);  
 }

The @Before method will use spring mock library to mock ServletContext and PageContext. The is good enough for simple pojo tag that does not rely on any application context.
What if out tag is depending on application context? An example would be a class referenced in the test implements InitializingBean interface. The use of the class is to load properties and initialize values. This requires spring context to be loaded.
To load Spring Application Context, such as WebApplicationContext, we need to add following lines in @Before method:

String configLocations = "/WEB-INF/applicationContext.xml,/WEB-INF/applicationContext-mock.xml";
 mockServletContext.addInitParameter(ContextLoader.CONFIG_LOCATION_PARAM, configLocations);
 ContextLoader loader = new ContextLoader();  
 loader.initWebApplicationContext(mockServletContext);

We first set the location of applicationContext file. Note that we add a applicationContext-mock.xml file to override beans defined in default context. This is handy because some beans may require external resource such as data source in order to be loaded correctly. We can override these beans to local pojo bean for the sake of unit testing.

Usually, application context is common to all tests so we only need to load spring context once before executing all tests. We can do so by adding these lines in @BeforeClass method. So our final setup looks like below:

@BeforeClass
 public static void init(){
  // mock ServletContext
  mockServletContext = new MockServletContext();
  //Add the following lines if your test depends on spring context to be loaded
  //For example, you have a referenced class that implements   
  org.springframework.beans.factory.InitializingBean interface
  String configLocations = "/WEB-INF/applicationContext.xml,/WEB-INF/applicationContext-mock.xml";
  mockServletContext.addInitParameter(ContextLoader.CONFIG_LOCATION_PARAM, configLocations);
  ContextLoader loader = new ContextLoader();  
  loader.initWebApplicationContext(mockServletContext);
 }
 
 @Before
 public void setup(){
  // mock PageContext
  mockPageContext = new MockPageContext(mockServletContext);  
  tag = new MyTag();
  tag.setPageContext(mockPageContext);  
 }

The test setup as above to load application context at the beginning of all test run. Then every test will create its own PageContext to hold the request and response as well as a new tag instance.

After setting up the test environment, it's time to write our test:

@Test
 public void testDoStartTag() throws Exception{  
 try{
  tag.setName("John");
  tag.doStartTag();
  String output = ((MockHttpServletResponse)mockPageContext.getResponse()).getContentAsString(); 
  assert("Hello John

".equals(output));
 }catch(JspException je){
  assert(false);
 }
}

The output string contains the actual http response (e.g. generated html code) of the tag when calling doStartTag method. So you can right specific assertion logic against the output of the tag.


The complete test class source is shown below:

public class TestDataInquiryTag {  
 @BeforeClass
 public static void init(){
  // mock ServletContext
  mockServletContext = new MockServletContext();
  //Add the following lines if your test depends on spring context to be loaded
  //For example, you have a referenced class that implements 
  //org.springframework.beans.factory.InitializingBean interface
  String configLocations = "/WEB-INF/applicationContext.xml,/WEB-INF/applicationContext-mock.xml";
  mockServletContext.addInitParameter(ContextLoader.CONFIG_LOCATION_PARAM, configLocations);
  ContextLoader loader = new ContextLoader();  
  loader.initWebApplicationContext(mockServletContext);
 }
 
 @Before
 public void setup(){
  // mock PageContext
  mockPageContext = new MockPageContext(mockServletContext);  
  tag = new MyTag();
  tag.setPageContext(mockPageContext);  
 }

 @Test
 public void testDoStartTag() throws Exception{  
  try{
   tag.setName("John");
   tag.doStartTag();
   String output = ((MockHttpServletResponse)mockPageContext.getResponse()).getContentAsString(); 
   assert("Hello John

".equals(output));
  }catch(JspException je){
   assert(false);
  }
 }
}