Extending JMeter

> ## Extending JMeter

> There are several ways to extend JMeter and add functionality. JMeter is designed to make this task easier.

> - A good overview of the process of extending JMeter

> - Creating your own Timer
> - Creating your own SampleListener (such as a visualizer, or reporter)
> - Creating your own Config Element
> - Creating your own logic SamplerController
> - Creating your own test sample SamplerController
> - Creating your own Sampler
> - Making your custom elements play nice as a JMeter UI component
> - Making your custom elements saveable and loadable from within JMeter

> ### Creating your own Timer

> The timer interface:

>     	  public long delay();

> Not too complicated. Your delay method must, each time it is called, return a long representing the number of milliseconds to delay. The constant timer returns the same number every time it's called. A random timer returns a different number each time.

> ### Creating your own SampleListener

> The SampleListener interface:

>     	  public void sampleOccurred(SampleEvent e);

>     	  public void sampleStarted(SampleEvent e);

>     	  public void sampleStopped(SampleEvent e);

> sampleOccurred is the method called when a sample is completed, and the data has been collected. The SampleEvent object should contain all the information gathered from the sample. If your sample listener is primarily concerned with collecting the data from a test run, you can implement this method - the other two are for other purposes and can be ignored (though the methods have to be there for your class to compile).

> sampleStarted and sampleStopped are used to indicate the state of the sampling thread. This is useful for visualizers that show the user the state of all running threads (ie, they are running and waiting for response, or they're stopped and waiting to begin again).

> ### Creating your own Config Element

> The ConfigElement interface:

>     	  public void addConfigElement(ConfigElement config);

>     	  public boolean expectsModification();

>     	  public Object clone();

> The ConfigElement interface is sparse. All ConfigElements are expected to implement a public clone() method. The reason for this is that config elements will be cloned for each different sampling thread, and most will be cloned for each sample.

> If your config element expects to be modified in the process of a test run, and you want those modifications to carry over from sample to sample (as in a cookie manager - you want to save all cookies that gets set throughout the test), then return true for the expectsModification() method. Your config element will not be cloned for each sample. If your config elements are more static in nature, return false. If in doubt, return false.

> addConfigElement() is required so that config elements can be layered. For instance, let's say a user creates a URL entry that contains default values - they might use this to specify a server. Then, all their test samples configure individual test cases, but leave out the server field. This information is combined via the addConfigElement() method. Your custom config elements should do the right thing when this method is called. Normally, this involves ignoring such calls unless the passed in ConfigElement is of the same type as yours, and then only merging in values that are not already set in the object receiving the call (ie you probably don't want to overwrite any values).

> You may have noticed there's no specification on how to get the config information out of a ConfigElement. This raises the question, who is going to use it? At the end of the line, there will be a Sampler that will need the information held in your config element. The sampler that uses your config element needs to know more about the class than the rest of JMeter - that information is not part of this interface.

> If at all possible, extend AbstractConfigElement when creating your own. By doing so, and by following some simple rules, you will get cloning and saving to XML of your config element for free (as in, you don't have to do anything!). AbstractConfigElement stores all it's values in a Map, and provides getProperty and putProperty methods. Your config element can provide getXXX() and setXXX() methods, but these should delegate to getProperty() and setProperty() , probably using static Strings as keys in the Map.
> You can store any type of object, provided the objects are clonable and Saveable (Strings, Integer, Long, Double, Float are all good in this regard).

> One caveat - if your config element has been restored from file, all the values held in the Map will be String objects (except for elements that implement Saveable on their own), and you may have to do casting and parsing. Example: an Integer will have to be converted from a String to an int, so your getXXX() method should check for this possibility to avoid exceptions.

> ### Creating your own logic SamplerController

> The SamplerController interface looks as follows:

>     	  Entry nextEntry();

>     	  Collection getListeners();

>     	  void addSamplerController(SamplerController controller);

>     	  void addConfigElement(ConfigElement config);

>     	  Object clone();

> Again, clone() is a method that must be implemented to all SamplerControllers to avoid contamination between sampling threads.

> The nextEntry() method is the essential job of a SamplerController - to deliver Entry objects to be sampled. An Entry object encapsulates all the information needed by a Sampler to do its job. The nextEntry() method should work like an iterator and continuously return new Entry objects.

> There are two boundary conditions that need to be handled. If the Controller has no more Entries to give, for the rest of the test, it should return null . Therefore, if your Controller has sub-controllers it is receiving Entries from, it should remove them from its list of controllers to get Entries from. The other condition is when your controller reaches the end of its list of Entries, and it needs to start over from the beginning. The parent Controller needs to know this so that it can move on to its next controller in its list. Therefore, at the end of each iteration, your SamplerController needs to return a CycleEntry object instead of a normal Entry. Conversely, this means that if your Controller receives a CycleEntry object, it should move on to the next Controller in its list.

> A logic controller does not generate Entries on its own, but simply regulates the flow of Entries from its sub-controllers. A logic controller might provide looping logic, or it might modify the Entries that pass through it, or whatever. GenericController provides an implementation that does absolutely nothing but pass Entries on from its sub-controllers. This class is useful both for reference purposes and to extend, since it provides a lot of methods you're likely to find useful

> getListeners() is an odd member of this Class. It's there to serve those who want their controller to receive sample data. This would be useful for a controller that modified Entry objects based on previous sample results (like an HTML spider that dynamically reacted to previously sampled webpages for links and forms). The responsibility of the controller implementer is to collect all potential listeners from the sub-controller list, and add themselves if desired. Most SamplerControllers that extend GenericController don't have to do anything.

> addSamplerController(SamplerController controller) is the method used to add sub controllers to your SamplerController.

> addConfigElement(ConfigElement config) Your SamplerController should also be capable of holding configuration elements and adding them to Entries as they pass through your controller. Again, see GenericController for reference. Essentially, all Entry objects that get returned by nextEntry() are handed all the ConfigElements of the controller.

> ### Creating your own test sample SamplerController

> A SamplerController that generates Entry objects is just like a logic controller except that it creates its own Entry objects instead of gathering them from sub-controllers (although, to be fully correct, your test sample SamplerController should handle both possibilities). Your test sample SamplerController can also benefit from extending GenericController. By doing so, most of your cloning and saving needs are handled (but probably not entirely). See HttpTestSample as reference.

> ### Creating your own Sampler

> The Sampler interface:

>     	  public SampleResult sample(Entry e)

> Your Sampler has two responsibilities. Of lesser importance, it should do whatever it is you want to do, given an Entry object that hopefully contains information about what is to be sampled. Of greater importance, your sampler should return a SampleResult object that holds information about the sampling. Information such as how long the sample took, the text response from the sample (if appropriate), and a string that describes the location of what was sampled. The SampleResult interface is essentially a Map with public static Strings as keys.

> ### Making your custom elements play nice as a JMeter UI component

> In order to take part in the JMeter UI, your component needs to implement the JMeterComponentModel interface:

>     	  Class getGuiClass();

>     	  public String getName();

>     	  public void setName(String name);

>     	  public Collection getAddList();

>     	  public String getClassLabel();

>     	  public void uncompile();

> Most of this stuff is easy, boring, and tedious. getName() , setName() is a simple String property that is the name of the object. getClassLabel() should return a String that describes the class. This string will be displayed to the user and so should be short but meaningful. getGuiClass() should return a Class object for the class that will be used as a GUI component. This class should be a subclass of java.awt.Container, and preferably a subclass of javax.swing.JComponent .

> getAddList() should return a list of either Strings or JMenus. These Strings represent the Classes that can be added to your SamplerController. Each String should correspond to the target class's getClassLabel() String. MenuFactory is a class that will return some preset menu lists (such as all available SamplerControllers, all available ConfigElements, etc).

> uncompile() is a cleanup method used between sampling runs. When the user hits "Start", JMeter "compiles" the objects in the tree. Child nodes are added to their parent objects recursively until there is one TestPlan object, which is then submitted for testing. Afterward, these elements have to un-added from their parent objects, or uncompiled. To uncompile your class, simply clear all your data structures that are holding sub-elements. For your SamplerController, this will be the list of sub-controllers and the list of ConfigElements.

> That's it, except for your GUI class. If your SamplerController has no configuration needs, just return org.apache.jmeter.gui.NamePanel, and the user will at least be able to change the name of your component. Otherwise, create a gui class that implements the ModelSupported interface:

>     	  void setModel(Object model);

>     	  public void updateGui();

> setModel() is used to hand your JMeterModelComponent class to the GUI class when it is instantiated. It is your responsibility for providing the means by which the Gui class updates the values in the model class. For updating in the other direction, there is updateGui() , which the model class can call if necessary. Note, normally, this call is made for you automatically whenever the Gui is brought to the screen. If you are creating a Visualizer, then you may need to use updateGui() . For reference, refer to UrlConfigGui (in org.apache.jmeter.protocol.http.config.gui).

> If you have done all this correctly, there's just one more step. If you compile your classes into the ApacheJMeter.jar file, then you're done. Your classes will be automatically found and used. Otherwise, you will need to modify jmeter.properties. The search_paths property should be modified to include the path where your classes are. This does not obviate the need for your classes to be in the JVM's CLASSPATH - it is an additional requirement. Otherwise, your classes will not be detected, and the Gui will not make them available to the user.

> ### Making your custom elements saveable and loadable from within JMeter

> The Saveable interface has just one method:

>     	  public Class getTagHandlerClass()

> This method simply returns the Class object that represents the Class that handles the saving and loading of your component.

> To write this SaveHandler, make a class that extends TagHandler (from org.apache.jmeter.save.xml). Note, if your component extends AbstractConfigElement, it is already fully Saveable - provided you only have information stored in the Map from AbstractConfigElement.

> To write your own TagHandler, you will have to implement the following methods:

>     	  public abstract void setAtts(Attributes atts) throws Exception

>     	  public String getPrimaryTagName()

>     	  public void save(Saveable objectToSave,Writer out) throws IOException

> getPrimaryTagName() should return the String that is the XML tagname that your class handles. When you save your object, it should all be contained within an XML tag of the same name. This will ensure that when JMeter's parser hits that tag, your class will be called upon to handle the data.

> setAtts(Attributes atts) is called when the parser first hits your tag. If this primary tag has any attributes, this method represents your chance to save the information.

> save(Saveable objectToSave,Writer out) - when the user selects "Save", JMeter will call this method and hand the Saveable object to be saved (it will be the object that specified your TagHandler as the class responsible for it's saving). This method should use the given Writer object to print all the XML necessary to save the current state of the objectToSave.

> There's more you have to do to handle creating a new Object when JMeter parses an XML file. However, there's no standard interface you need to implement, but rather, JMeter uses reflection to generate method calls into your class. When JMeter hits a tag that corresponds to your PrimaryTagName, an instance of your TagHandler will be created, and it's setAtts() method will get called. Thereafter, methods are called depending on subsequent tags and character data. For every tag, JMeter calls TagStart(Attributes atts) , and for every end tag, JMeter calls TagEnd() .

> Additionally, JMeter will call a method that corresponds to all tags that are current. So, for instance, if JMeter runs into a tag name "foo", then foo(Attributes atts) will be called. If JMeter then parses character data, then foo(String data) will be called. If JMeter parses a tag within foo, called "nestedFoo", then JMeter will call foo_nestedFoo(Attributes atts) and foo_nestedFoo(String data) . And so on.

> An annotated example:

>     public class AbstractConfigElementHandler extends TagHandler

>     {

>     	private AbstractConfigElement config;

>     	private String currentProperty;

>     	public AbstractConfigElementHandler()

>     	{

>     	}

>     								/**

>     	 * Returns the AbstractConfigElement object parsed from the XML.  This method

>     	 * is required to fulfill the SaveHandler interface.  It is used by the XML

>     	 * routines to gather all the saved objects.

>     	 */

>     	public Object getModel()

>     	{

>     		return config;

>     	}

>     								/**

>     	 * This is called when a tag is first encountered for this handler class to handle.

>     	 * The attributes of the tag are passed, and the SaveHandler object is expected

>     	 * to instantiate a new object.

>     	 */

>     	public void setAtts(Attributes atts) throws Exception

>     	{

>     		String className = atts.getValue("type");

>     		config = (AbstractConfigElement)Class.forName(className).newInstance();

>     	}

>     								/**

>     	 * Called by reflection when a <property> tag is encountered.  Again, the

>     	 * attributes are passed.

>     	 */

>     	public void property(Attributes atts)

>     	{

>     		currentProperty = atts.getValue("name");

>     	}

>     								/**

>     	 * Called by reflection when text between the begin and end <property>

>     	 * tag is encountered.

>     	 */

>     	public void property(String data)

>     	{

>     		if(data != null && data.trim().length() > 0)

>     		{

>     			config.putProperty(currentProperty,data);

>     			currentProperty = null;

>     		}

>     	}

>     								/**

>     	 * Called by reflection when the <property> tag is ended.

>     	 */

>     	public void propertyTagEnd()

>     	{

>     								// Here's a tricky bit.  See below for explanation.

>     		List children = xmlParent.takeChildObjects(this);

>     		if(children.size() == 1)

>     		{

>     			config.putProperty(currentProperty,((TagHandler)children.get(0)).getModel());

>     		}

>     	}

>     								/**

>     	* Gets the tag name that will trigger the use of this object's TagHandler.

>     	*/

>     	public String getPrimaryTagName()

>     	{

>     		return "ConfigElement";

>     	}

>     								/**

>     	* Tells the object to save itself to the given output stream.

>     	*/

>     	public void save(Saveable obj,Writer out) throws IOException

>     	{

>     		AbstractConfigElement saved = (AbstractConfigElement)obj;

>     		out.write("\n");

>     		Iterator iter = saved.getPropertyNames().iterator();

>     		while (iter.hasNext())

>     		{

>     			String key = (String)iter.next();

>     			Object value = saved.getProperty(key);

>     			writeProperty(out,key,value);

>     		}

>     		out.write(");

>     	}

>     								/**

>     	 * Routine to write each property to xml.

>     	 */

>     	private void writeProperty(Writer out,String key,Object value) throws IOException

>     	{

>     		out.write("\n");

>     		JMeterHandler.writeObject(value,out);

>     		out.write("\n\n");

>     	}

> In the propertyTagEnd() method, takeChildObjects() is called on the xmlParent instance variable. xmlParent is inherited from TagHandler - the DocumentHandler object that is running the show. xmlParent takes an XML file that represents a portion of the test configuration tree, and recreates a tree-like data structure. When it is done, it will convert its tree-like data structure into the test configuration tree structure.

> However, sometimes, a tree element has sub objects that you do not want represented in the tree - rather, they are part of your object. But, they may be complicated enough to warrant their own SaveHandler class, and thus, the xmlParent picks them up as part of its tree. When the tag is done, and you know that there are child objects you want to grab, you can call the takeChildObjects() method and get a List object containing them all. This will remove them from the tree, and you can add them to your object that you're creating.

> UrlConfig is good example. It extends AbstractConfigElement, so it uses exactly the code above to save and reload itself from XML. However, one of the pieces of data that UrlConfig stores is an Arguments object. Arguments is too complicated to save to file as a simple string, so it has its own Handler object (ArgumentsHandler). In the above code, when the call to JMeterHandler.writeObject(value,out) is made, the writeObject method detects whether the object implements Saveable, and if so, calls the object's SaveHandler class to deal with it. This means, however, that when reading that XML file, the Argument object will show up as a separate entity in the data tree, whereas it originally was just part of the data of the UrlConfig object. In order to preserve that relationship, it's necessary for the AbstractConfigElementHandler to check after each property tag is done for child objects in the tree, and take them for its own use.

> Study the other SaveHandler objects and the TagHandler class to learn more about how saving is accomplished. Once you understand the design, writing your own SaveHandler is very easy.

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