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Hotreload with tests. playframework how?

So Im currently in a project where we are using Java playframework 2.3.7 with activator.
One of the things I liked about playframework is the hot-reloading feature. I can modify java files save and the changes are compiled and refreshed on runtime.
How do I get that functionality but for testing? I want to be able to run a single test with this hot reloading feature, so that when I save. Tests for the given file (specified by test-only) is re-runned automatically.

There is not such a solution, however you have two choices:
Use IntellJ: To re-run the previous test(s) in IntellJ, you press shift + F10.
Write a watcher: Write a file/directory watcher such as this question/answer here, and then as soon as there are changes, the program, re-runs the test command, such as sbt clean compile test or activator compile test.
Little advice auto running tests: I don't know how complicated your application is, but as soon as you have couple of injections here and there and with additional concurrency; you do not want to run the tests as soon as you put a char in.
Little advice on Test Driven Development: Your approach should be the other way around! You write a test, which fails because there is no implementation; then you leave it alone. You go and write the implementation, then rerun the test to pass it or to get a feedback. Again, you need your cpu/memory power to focus on one thing, you don't want to brute force your implementation. Hope this makes sense!.
Little advice on your Play version: The Play 2.6 is way much better than Play 2.3; you should slowly but surely update your application; at least for the sake of security.

Ok so I found what I was looking for.
For anybody in need of this particular feature in that particular version of play (I'm not sure about other versions) what you need to do is really simple. run activator and put the ~ prefix before test. for example
#activator
[my-cool-project]~test
That will reload your tests when you make a change. if you want to do this for a particular test then you have to do the same but with test-only
#activator
[my-cool-project]~test-only MyCoolTest
hope it helps anyone looking for the same thing

java.lang.StackOverflowError when parsing Groovy script on Jenkins

We are experiencing a problem with our Jenkins CI server.
Our CI implementation relies on several Groovy scripts, which we execute in Jenkins as "System Groovy scripts". This has been this way for years, and the scripts have undergone no recent modifications, and implement build flows, business logic steps such as version checking, etc.
Yesterday we started experiencing an exception in every Jenkins job that we tried to lauch that, one way or another, tried to execute Groovy scripts. The exception is:
java.lang.StackOverflowError
at org.codehaus.groovy.antlr.parser.GroovyRecognizer.additiveExpression(GroovyRecognizer.java:12478)
at org.codehaus.groovy.antlr.parser.GroovyRecognizer.shiftExpression(GroovyRecognizer.java:9695)
at org.codehaus.groovy.antlr.parser.GroovyRecognizer.relationalExpression(GroovyRecognizer.java:12383)
at org.codehaus.groovy.antlr.parser.GroovyRecognizer.equalityExpression(GroovyRecognizer.java:12307)
at org.codehaus.groovy.antlr.parser.GroovyRecognizer.regexExpression(GroovyRecognizer.java:12255)
at org.codehaus.groovy.antlr.parser.GroovyRecognizer.andExpression(GroovyRecognizer.java:12223)
at org.codehaus.groovy.antlr.parser.GroovyRecognizer.exclusiveOrExpression(GroovyRecognizer.java:12191)
hundreds of similar lines
at org.codehaus.groovy.antlr.parser.GroovyRecognizer.compoundStatement(GroovyRecognizer.java:7510)
at org.codehaus.groovy.antlr.parser.GroovyRecognizer.compatibleBodyStatement(GroovyRecognizer.java:8834)
at org.codehaus.groovy.antlr.parser.GroovyRecognizer.statement(GroovyRecognizer.java:899)
at org.codehaus.groovy.antlr.parser.GroovyRecognizer.compilationUnit(GroovyRecognizer.java:757)
at org.codehaus.groovy.antlr.AntlrParserPlugin.transformCSTIntoAST(AntlrParserPlugin.java:131)
at org.codehaus.groovy.antlr.AntlrParserPlugin.parseCST(AntlrParserPlugin.java:108)
at org.codehaus.groovy.control.SourceUnit.parse(SourceUnit.java:236)
at org.codehaus.groovy.control.CompilationUnit$1.call(CompilationUnit.java:161)
at org.codehaus.groovy.control.CompilationUnit.applyToSourceUnits(CompilationUnit.java:846)
at org.codehaus.groovy.control.CompilationUnit.doPhaseOperation(CompilationUnit.java:550)
at org.codehaus.groovy.control.CompilationUnit.processPhaseOperations(CompilationUnit.java:526)
at org.codehaus.groovy.control.CompilationUnit.compile(CompilationUnit.java:503)
at groovy.lang.GroovyClassLoader.doParseClass(GroovyClassLoader.java:302)
at groovy.lang.GroovyClassLoader.parseClass(GroovyClassLoader.java:281)
at groovy.lang.GroovyShell.parseClass(GroovyShell.java:731)
at groovy.lang.GroovyShell.parse(GroovyShell.java:743)
at groovy.lang.GroovyShell.parse(GroovyShell.java:770)
at groovy.lang.GroovyShell.parse(GroovyShell.java:761)
at groovy.lang.GroovyShell$parse.call(Unknown Source)
at com.cloudbees.plugins.flow.FlowDSL.executeFlowScript(FlowDSL.groovy:80)
at com.cloudbees.plugins.flow.FlowRun$FlyweightTaskRunnerImpl.run(FlowRun.java:219)
at hudson.model.Run.execute(Run.java:1759)
at com.cloudbees.plugins.flow.FlowRun.run(FlowRun.java:155)
at hudson.model.ResourceController.execute(ResourceController.java:89)
at hudson.model.Executor.run(Executor.java:240)
at hudson.model.OneOffExecutor.run(OneOffExecutor.java:43)
This looks like that the Groovy parser inside Jenkins is reaching the top of the stack while trying to parse the groovy script (as I have said, this abruptly started to happen with many scripts that worked perfectly before and had undergone no recent modification).
Currently our Jenkins installation (v1.594) runs on a Websphere 8.5.5.2 application server on AIX v7.1 (don't know exactly the fix pack level and / or if it has recently suffered any kind of update, still trying to gather the info).
After a restart, we returned to normal behavior (all the scripts were working as usual again without any modification to them).
Does anyone know about some incompatibility of any underlying library with Jenkins Groovy parsing?

There is a problem with the groovy code; causing the parser to go nuts:
java.lang.StackOverflowError
at org.codehaus.groovy.antlr.parser.GroovyRecognizer.additiveExpression(GroovyRecognizer.java:12478)
Based on a similar ticket:
https://issues.apache.org/jira/browse/GROOVY-1783,
it is possible that your code has circular references; or creating too many functions on the fly. You can take the approach of analyzing your code and trying to put anything that is going to make allocations outside of loops; in particular complex inline functions.
Another approach is to go look at the Build Flow plugin and scroll down the documentation and see how you could write an extension point rather than use groovy. This may not be easy to do and requires effort; but you can write a lot of tests for your code that way. You would still use groovy for the glue; but use java directly for the hot spots.
A third approach would be to file a ticket on the Groovy issue tracker; and see what the experts find out.

Java project with executable jars; running and retrieving data from them

At work, I use a Java application (I have located compiled/executable jars on the C-drive). I want to be able to grab some information from this application through code. The application itself probably does not store information, so it must communicate with legacy systems some way, I am not sure how, I have seen traces of a Servlet(?) Hence, I suspect the application also has built-in "encryption"(?)
I do not want to get involved in encryption and login procedures etc., so I am thinking I could just build a Java project around the current executable jars, and launch the application as I usually do (through the "main" entry point, "Start.jar", but then after execution call the functions that I want to (i.e. the application just runs as usual in the background)...
Would that be possible? Is there another way? Can one, for example, hook up to an already executed Java application and issue commands?
What I have tried so far
Downloaded Eclipse, and created a new project
Made Eclipse "reference" external jars (there was a wizard in Eclipse)
Created a new class in my new project, in which I launch the "main" entry point of the "main" executable jar (the structure of all the jars pops up with "IntelliSense"). I have also found out which argument I need to supply to the main procedure using JD-GUI (Java Decompiler)...
It seems that from inside the main procedure a call is made to another procedure, which resides in a different jar, in the debug window of Eclipse I just see an error, which made me doubt that my current approach is viable... Maybe the problem arises because the command is issued from a compiled jar? Could there be an issue with the "class path"? Does this at all seem like a solution? But then again, I have no experience with Java (mostly VBA and some C#).

You can start your JVM for the application with options, which enable remote debugging. Then you can connect the eclipse debugger to this JVM.
http://www.eclipsezone.com/eclipse/forums/t53459.html

Based on your question, I am going to guess that your application does not have a Java API you can code against. That would, of course, be the easiest way. So, if you have not checked, do that first.
Assuming you don't have an API to code against, I think your approach is correct. But it could be hard to do, since you are basically flying blind trying to figure out what the application is doing. Remote debugging might solve part of that problem.
There might be a slightly easier solution, if you are sure it is sending requests across the network. You can use a tool like Wireshark to see what it is creating. Then, you can have your application create requests that look like that and send them to that destination. This assumes of course that the requests aren't encrypted. In that case you are probably out of luck.

Play! Java web framework. How does their development server compile automatically?

After looking at the Play! framework I find it really productive that the development server that it comes with automatically is able to compile .java files and show the changes, immediately. There's no hot deployer scanner that runs every tot seconds or so. The compilation happens when you hit refresh and it's extremely faster than my incremental mvn package. How do they do this?
I would like to know, well because I'm interested in knowing, but also because I don't want to use the entire Play! framework for my small project. I'm only interested in their development compilation process because I would like to adopt it :).
Any ideas?

I was reading about this just this morning. It actually takes your changed source files and uses the Eclipse Java Compiler (ECJ) internally before spitting out the compiled files to the built-in dev server.
The thing is, you probably don't want to go to the effort of wiring the ECJ into your "small project".
You can definitely approximate it though - the trick is to not do a mvn package, instead you want to be dropping the changed .class files into your webapp's exploded warfile directory on the filesystem.
If you're not tied to a particular app server/container, have a look at the Eclipse Jetty Plugin - looks like it's what you need, and Jetty is quick

JDT -- Play! uses Eclipse JDT to compile and load classes dynamically. Much the same way you code in Eclipse and you see an error or warning messages as soon as you type in something not desirable. See ApplicationCompiler class.
You may also want to look into JDT.

Play uses the Eclipse compiler to compile code at run-time.
Take a look at the following class, that is used by Play to perform the necessary compilation at run time.
https://github.com/playframework/play/blob/master/framework/src/play/classloading/ApplicationCompiler.java

The way they do it is by using a custom classloader that will detect changes to source files, use the Eclipse Java Compiler to compile the files and then hot swap the appropriate classes in the JVM. If you are looking for something similar, checkout ZeroTurnaround's JRebel
it is not free, but well worth the time savings when you need to redeploy a large project.

I'm not a Play developer however Struts2 is also capable of this but though the struts2-spring-plugin. Since the class reloading is provided by Spring it might be possible to use this spring feature by any project.
http://struts.apache.org/2.2.1/docs/spring-plugin.html
Search the page for "Class Reloading".

How to compile single/multiple java files without server restart? Is there any Eclipse plugin for the same?

I want to compile multiple java files in my Application without restarting Weblogic and Tomcat. Otherwise, this takes a lot of time. For this I got one Hotswap plugin in Eclipse, but this is not working in all the cases. It says it works for single file compile. Even if I use that it is not working in all the cases.
Is there any other way that I can do this or is there any other plugin/script/software which can help me in this?
If there are some open source ones, it will be very helpful.
Thanks in Advance.

One thing is compiling the classes, but you also need the JAVA VM to reload the classes and use them, which is called hot-swapping. For the best available hot-swapping of classes you'll need something like javarebel. It allows you to hot-reload a lot more types of code-changes than the regular SUN JVM. If you run your deployment in exploded-mode you're home free, and can test any code change in a second or so. We're fairly test-driven, so I only use javarebel in that short phase when I assemble the whole application, but it works really well.

The Java HotSpot VM does this automatically, but not in all cases...
First, you must be running a "debug" session, not a "run" session.
Next, some changes will force a restart. The basic idea is that if the interface to the class change (the method sigs, not an actual Java interface), the VM can't replace the class contents inline.
You shouldn't need a plugin for this, just a recent-ish VM.
This happens under several circumstances like
adding methods
removing methods
changing method signatures
changing the class hierarchy (superclasses, implemented interfaces)
It can also happen if you introduce an error into the class. (For errors like mismatched curly braces)
When you save a Java file, eclipse compiles it. If there is an error, eclipse inserts an exception throw to indicate that there is an unresolved compilation error. (It does this so when you run you don't just see the last successful compilation, making it obvious you have a compiler error)

I don't know Eclipse, but I do use Netbeans. Netbeans does this pretty well. The latest version even has an option to automatically recompile when you save a java file.
I know this doesn't exactly answer your question. You can probably use both Netbeans and Eclipse depending on what part of the project you're working on.
EDIT:
With Tomcat you can reload the web app. This is really only useful if Tomcat is looking at the new class. If your project is compiled to a build directory first and a WAR is then created from this you can go into Tomcat and install the web app and instead of pointing at a WAR point at the build directory.
In Tomcat you may have to put a site config file under tomcat/conf/Catalina/localhost. The contents of this file looks like this:
<?xml version="1.0" encoding="UTF-8"?>
<Context docBase="C:/Projects/MyWebApp/build/web" path="/MyWebApp"/>
Instructions for reloading here:
http://www.cs.ucl.ac.uk/teaching/java/tomcatfaq.html#changeservlet
If you do this a few times though Tomcat will run out of memory. This is because of something called PermGenSpace. Read up about it if you want to know more. The solution is to increase the JVM memory, the PermGenSize (with -XX:MaxPermSize) and finally restarting Tomcat occasionally.
EDIT2:
If reloading the app causes you to be logged out you may be able to easily get the container to serialize your session data to disk by adding 'implements Serializable' to some of your classes. Then you should not need to login after reloading the app.

I agree that it is very tedious to redeploy all the time when developing.
I would suggest you look into MyEclipse which has a very good hotdeploy mechanism which works well with Tomcat, and which is quite affordable and has a 30 day trial.
The stock Java EE mechanism in Eclipse for redeploying to servers is nowhere as fast.

I guess I don't really see where the problem is. Here is what I do and the changes load almost instantaneously. I have an Ant script that compiles the .java and .jsp files for me, puts them in the appropriate directories under webapps and changes the web.xml file if necessary (or at least touches it to notify tomcat of the changes). If you need help on doing any of that with Ant, I'd be happy to help. Btw I do not use WAR files for deployment on my testing machine. That would be a lot slower I guess.

very easily done if you read this page:
See http://blog.redfin.com/devblog/2009/09/how_to_set_up_hot_code_replacement_with_tomcat_and_eclipse.html
Thank you Dan Fabulich