JarCastingEver wondered while using JSTL tags during the developing of a JSP page what content you actually can use within an expression? Take a look at the following example:
<html>
...
<c:out value="${flight.destination.airportCode}" />
...
</html>
Okay, so you print the airport code of the destination of a flight. So far, so good. But what if you want to print other information about the flight as well? To know what information is available in the flight object you either have to take a look into the actual source code for the Java class of the object you're interested in (here: the flight object) or - if you don't have access to the source code - find someone who can give you that information. In either case it's not as quick and easy as you'd like.
That's were the BeanDumper comes into play. It's a nice little JSTL tag that prints the object graph containing all the properties of an object (and the properties of the properties, and so on).
It's usage is quite simple and involves two steps. First, you have to include the dependency to the pom.xml of your project:
<project>
...
<dependency>
<groupId>de.perdian.apps.beandumper</groupId>
<artifactId>beandumper</artifactId>
<version>1.1.0</version>
</dependency>
...
</project>
Now all you have to do in your JSP file is declare the tag library and insert the tag at the corresponding location where you want the output to be shown:
<%@ taglib prefix="bd" uri="http://github.com/perdian/beandumper" %>
<html>
...
<div id="somewhere-in-your-page">
<bd:dump root="${flight}" prefix="flight" />
</div>
...
</html>
Now, the resulting page will output the complete object graph of the flight object.
flight = ...
flight.destination = ...
flight.destination.airportCode = MUC
flight.destination.current = <null>
flight.destination.date = 18. Okt 2013
flight.destination.gate = A42
flight.destination.scheduled = 08:05
flight.destination.status = cancelled
flight.destination.terminal = <null>
flight.flightNumber = 1234
flight.operatingCarrier = <null>
flight.origin = ...
flight.origin.airportCode = DUS
flight.origin.current = <null>
flight.origin.date = 18. Okt 2013
flight.origin.gate = A42
flight.origin.scheduled = 06:55
flight.origin.status = cancelled
flight.origin.terminal = <null>
The left part of the each line represents the key that can be used within the JSTL expression, the right part represents the value that is stored inside the beans property. So, in our example, if you want to print the status of the destination gate you could directly copy and pase the expression into your page:
<html>
...
<div>Destination airport: <c:out value="${flight.destination.airportCode}" /></div>
<div>Destination gate: <c:out value="${flight.destination.gate}" /></div>
...
</html>
You might want to know why you have to enter the flight key twice into the tag. This has to do with the way JSP handles object references. The ${flight} tag will be converted internally to the actual object, which means the tag itself doesn't know that the object it received was originally named "flight". So, we also have to pass the name to the tag as prefix. It is explicitely called prefix because the name is just added to the front of the key. We could also have set the prefix to "foo" and then the output keys from the list above would have been like "foo.flightNumber = XY1234". The tag doesn't care, but then you wouldn't be able to just copy and paste the key - so keep in mind to keep the "root" and the "prefix" attributes values in sync.
Bonus feature: If you completly omit the "root" and "prefix" attributes, then the tag will print the complete context, which means all the objects that you can you in the JSP page - not just a single one you're interested in like we saw in the first example.
<%@ taglib prefix="bd" uri="http://github.com/perdian/beandumper" %>
<html>
...
<div id="bottom-of-your-page">
<bd:dump />
</div>
</html>
If you want to use this feature you should place the tag at the very bottom of your page since the output might become quite large.
You can also add ignore pattern, which define which parts of the output you do not want to see in the result:
<bd:dump>
<bd:ignore pattern=".*?de\.test\..*" />
</bd:dump>
This example will remove any output from the result that starts with de.test.. Please be aware, that once an object has been removed, it's children will not be added to the output as well. For example, take the following regular output:
flight = ...
flight.destination = ...
flight.destination.airportCode = MUC
If we now create the following ignore pattern:
<bd:dump>
<bd:ignore pattern="flight\.destination" />
</bd:dump>
While strictly speaking this should just remove the second line from the output, it will also remove the third line flight.destination.airportCode from the output, since the airportCode property is a direct descendent from the flight.destination object and once an object has been removed from the result all it's child properties will also never be accessed.
Note: The output that is generated by the tag doesn't always has to be exactly what you'll see when using the same expression in a c:out or other tag. For regular Java types (like String, Integer, ...) you can expect it to be very acurate. However, if you're dealing with complex objects that have very special behaviour then you might see different output results.
Always remember: The dump tag isn't designed to be a 1:1 mirror of what a JSP evaluation will produce - it's just a simple and quick way to get information about an object within the JSP context.