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Java: find classes with certain base type on program startup

I have a Java program (an Eclipse plug-in to be specific), which is using several JAR files (which I can control). One of the core JAR files defines an abstract class called Strategy. When the user starts the program, the program needs to know all sub classes of Strategy, that are on the program ClassPath.
As I described in this StackOverflow question, I tried to use static initializer blocks on the sub classes, so that they auto-register themselves in a registry. This approach is not working since the static initalizers are not executed before I explicitly use the class.
Is there any other way to find all classes with a certain base type that are on the current ClassPath?
I can think of the following solutions:
traverse all JARs in a specific directory and check the classes they contain
create a file which is loaded on program start-up and read the class names from that
I can implement both of them myself (so I do not ask for any code samples here). I merely want to know if there is another option I am missing. Any in-class solution (like the one I tried with static initializers) would be greatly appreciated.

If you're open to using a third-party library, Reflections seems to be the best fit here. It's a Java runtime metadata analysis library and is like geared to do this. From their website:
Using Reflections you can query your metadata such as:
get all subtypes of some type
get all types/constructos/methods/fields annotated with some annotation, optionally with annotation parameters matching
get all resources matching matching a regular expression
get all methods with specific signature including parameters, parameter annotations and return type
All you need is to create a configured instance of Reflections in your StrategyRegistrar class as
Reflections reflections = new Reflections(
new ConfigurationBuilder()
.setUrls(ClasspathHelper.forPackage("com.your.app.strategies.pkg"))
.setScanners(new SubTypesScanner())
);
And, then simply fire a query like
Set<Class<? extends Strategy>> strategies =
reflections.getSubTypesOf(com.your.app.strategies.pkg.Strategy.class);

Make a Java class have higher compile priority than one inside a library in IDEA

We have some portion of functionality packed in an external library and it is attached to our project. That library can't be changed in any way. Amongst others there are two classes lying inside it: com.myorg.Grandpa and com.myorg.Dad that extends com.myorg.Grandpa. Also there are com.myorg.Grandson extending com.myorg.Dad and a few other classes outside of the library extending com.myorg.Grandpa.
I decompile com.myorg.Grandpa class and add a new method new_method() to it.
Then I try to use new_method() in com.myorg.Grandson but IDEA won't let me do it cause Grandson extends Dad which extends library's Grandpa which doesn't contain new_method().
I tried to delete Grandpa from library and surprisingly IDEA didn't say a word and successfully compiled a project despite of the fact that in the boundaries of a library Dad extends non existing class.
The question is how to force Dad to extend a new Grandpa without deleting the one inside a library?

You could
Add an abstract class between Dad and GrandSon: Extend Dad, and add your method in the sub class. Then derive GrandSon from that sub class.
Put an instance of Dad in a new class, and let your IDE create delegate methods to the aggregated Dad instance. Again add your new method to the new class.
There is another possibility:
If you have to modify classes in place, use aspectj to weave in code: aspectj changes the byte-code (it does not need source code) at run-time. This way you can add methods or fields.

The fact is that you are duplicating classes with full package signature, so you will get the one that the classloader loads first. I know that in Websphere you can tweak classloader priorities, but couldn't say in your case.
Anyway, why not just do it without decompiling? You are causing yourself hard coupling to an external library and bad practices (maybe copyright issues) by decompiling/customizing. Besides, if the library gets updated, you will run into trouble having to reconstruct your customized classes.
Options:
Create your own implementation, for instance:
Create an Interface that replicates all methods in Grandpa plus the one you need.
Extend Grandpa class and implement the added method from your interface, all other methods will be left intact.
Extend all other extending classes from your own class hierarchie.
Instead of using the libraries common class, use your Interface as naming
This way you are kind of creating your own interface to the library, if it changes, you know where to make changes.
You could even do it without the interface, it's kind of wrapping the functionality, it would depend on what you need to achieve.
Anyway, I would try to solve it by own code and not by messing up with the library, it is just not worth it to do such tricks, if a new Programmer takes the project, they will need a lot of time to find out why and how it behaves.
Now, there might be variations in how to structure the class hierarchie, but it would depend on the specific implementation you need, so you would have to post more detailed data on what the library is and what you're trying to add to it if you expect some more specific answer...
Regards

It has to appear first to the class loader.
IDEA should load your class first if is in your project. You may also try to create a separate library for your class and include it in your project.
See also: http://www.jetbrains.com/idea/webhelp/configuring-module-dependencies-and-libraries.html

How to list all the classes on the class path that implements a given set of interfaces

Most IDEs allows you to do this easily for a single interface, but I am looking for classes that implements two interfaces at the same time. Is there anyway to automatically do this (without having to manually open every implementer and check)?
If no IDE supports, does Spring have such feature. (I know I can get a list of beans implementing a specific interface, but I am not sure if I can do it for all classes.)

from System.properties you will get the classpath.
TRY use TRY, to open it like a zip. (Inflater or Deflater stream) you will have the class
load the class with a classloader
use instanceof xInterface && instance of yInterface
upvote if it helped :)

Possibly org.reflections has what you need - unless I still misunderstand your requirements.

How can I run my code upon class load?

Is there a feasible way to get my own code run whenever any class is loaded in Java, without forcing the user explicitly and manually loading all classes with a custom classloader?
Without going too much into the details, whenever a class implementing a certain interface read its annotation that links it with another class, and give the pair to a third class.
Edit: Heck, I'll go to details: I'm doing an event handling library. What I'm doing is having the client code do their own Listener / Event pairs, which need to be registered with my library as a pair. (hm, that wasn't that long after all).
Further Edit: Currently the client code needs to register the pair of classes/interfaces manually, which works pretty well. My intent is to automate this away, and I thought that linking the two classes with annotations would help. Next, I want to get rid of the client code needing to keeping the list of registrations up to date always.
PS: The static block won't do, since my interface is bundled into a library, and the client code will create further interfaces. Thus, abstract classes won't do either, since it must be an interface.

If you want to base the behavior on an interface, you could use a static initializer in that interface.
public interface Foo{
static{
// do initializing here
}
}
I'm not saying it's good practice, but it will definitely initialize the first time one of the implementing classes is loaded.
Update: static blocks in interfaces are illegal. Use abstract classes instead!
Reference:
Initializers (Sun Java Tutorial)
But if I understand you right, you want the initialization to happen once per implementing class. That will be tricky. You definitely can't do that with an interface based solution. You could do it with an abstract base class that has a dynamic initializer (or constructor), that checks whether the requested mapping already exists and adds it if it doesn't, but doing such things in constructors is quite a hack.
I'd say you cleanest options are either to generate Code at build time (through annotation processing with apt or through bytecode analysis with a tool like asm) or to use an agent at class load time to dynamically create the mapping.
Ah, more input. Very good. So clients use your library and provide mappings based on annotations. Then I'd say your library should provide an initializer method, where client code can register classes. Something like this:
YourLibrary.getInstance().registerMappedClasses(
CustomClass1.class,
CustomClass2.class,
CustomClass3.class,
CustomClass4.class
)
Or, even better, a package scanning mechanism (example code to implement this can be found at this question):
YourLibrary.getInstance().registerMappedClassesFromPackages(
"com.mycompany.myclientcode.abc",
"com.mycompany.myclientcode.def"
)
Anyway, there is basically no way to avoid having your clients do that kind of work, because you can't control their build process nor their classloader for them (but you could of course provide guides for classloader or build configuration).

If you want some piece of code to be run on any class loading, you should:
overwrite the ClassLoader, adding your own custom code at the loadClass methods (don't forget forwarding to the parent ClassLoader after or before your custom code).
Define this custom ClassLoader as the default for your system (here you got how to do it: How to set my custom class loader to be the default?).
Run and check it.
Depending on what kind of environment you are, there are chances that not all the classes be loaded trouugh your custom ClassLoader (some utility packages use their own CL, some Java EE containers handle some spacific areas with specific classLoaders, etc.), but it's a kind of aproximation to what you are asking.

Find Java classes implementing an interface [duplicate]

This question already has answers here:
How can I get a list of all the implementations of an interface programmatically in Java?
(11 answers)
Closed 9 years ago.
Some time ago, I came across a piece of code, that used some piece of standard Java functionality to locate the classes that implemented a given interface. I know the functions were hidden in some non-logical place, but they could be used for other classes as the package name implied. Back then I did not need it, so I forgot about it, but now I do, and I can't seem to find the functions again. Where can these functions be found?
Edit: I'm not looking for any IDE functions or anything, but rather something that can be executed within the Java application.

Awhile ago, I put together a package for doing what you want, and more. (I needed it for a utility I was writing). It uses the ASM library. You can use reflection, but ASM turned out to perform better.
I put my package in an open source library I have on my web site. The library is here: http://software.clapper.org/javautil/. You want to start with the with ClassFinder class.
The utility I wrote it for is an RSS reader that I still use every day, so the code does tend to get exercised. I use ClassFinder to support a plug-in API in the RSS reader; on startup, it looks in a couple directory trees for jars and class files containing classes that implement a certain interface. It's a lot faster than you might expect.
The library is BSD-licensed, so you can safely bundle it with your code. Source is available.
If that's useful to you, help yourself.
Update: If you're using Scala, you might find this library to be more Scala-friendly.

Spring can do this for you...
BeanDefinitionRegistry bdr = new SimpleBeanDefinitionRegistry();
ClassPathBeanDefinitionScanner s = new ClassPathBeanDefinitionScanner(bdr);
TypeFilter tf = new AssignableTypeFilter(CLASS_YOU_WANT.class);
s.addIncludeFilter(tf);
s.scan("package.you.want1", "package.you.want2");
String[] beans = bdr.getBeanDefinitionNames();
N.B. The TypeFilter is important if you want the correct results!
You can also use exclusion filters here instead.
The Scanner can be found in the spring-context jar, the registry in spring-beans, the type filter is in spring-core.

I really like the reflections library for doing this.
It provides a lot of different types of scanners (getTypesAnnotatedWith, getSubTypesOf, etc), and it is dead simple to write or extend your own.

The code you are talking about sounds like ServiceLoader, which was introduced in Java 6 to support a feature that has been defined since Java 1.3 or earlier. For performance reasons, this is the recommended approach to find interface implementations at runtime; if you need support for this in an older version of Java, I hope that you'll find my implementation helpful.
There are a couple of implementations of this in earlier versions of Java, but in the Sun packages, not in the core API (I think there are some classes internal to ImageIO that do this). As the code is simple, I'd recommend providing your own implementation rather than relying on non-standard Sun code which is subject to change.

Package Level Annotations
I know this question has already been answered a long time ago but another solution to this problem is to use Package Level Annotations.
While its pretty hard to go find all the classes in the JVM its actually pretty easy to browse the package hierarchy.
Package[] ps = Package.getPackages();
for (Package p : ps) {
MyAno a = p.getAnnotation(MyAno.class)
// Recursively descend
}
Then just make your annotation have an argument of an array of Class.
Then in your package-info.java for a particular package put the MyAno.
I'll add more details (code) if people are interested but most probably get the idea.
MetaInf Service Loader
To add to #erickson answer you can also use the service loader approach. Kohsuke has an awesome way of generating the the required META-INF stuff you need for the service loader approach:
http://weblogs.java.net/blog/kohsuke/archive/2009/03/my_project_of_t.html

You could also use the Extensible Component Scanner (extcos: http://sf.net/projects/extcos) and search all classes implementing an interface like so:
Set<Class<? extends MyInterface>> classes = new HashSet<Class<? extends MyInterface>>();
ComponentScanner scanner = new ComponentScanner();
scanner.getClasses(new ComponentQuery() {
#Override
protected void query() {
select().
from("my.package1", "my.package2").
andStore(thoseImplementing(MyInterface.class).into(classes)).
returning(none());
}
});
This works for classes on the file system, within jars and even for those on the JBoss virtual file system. It's further designed to work within standalone applications as well as within any web or application container.

In full generality, this functionality is impossible. The Java ClassLoader mechanism guarantees only the ability to ask for a class with a specific name (including package), and the ClassLoader can supply a class, or it can state that it does not know that class.
Classes can be (and frequently are) loaded from remote servers, and they can even be constructed on the fly; it is not difficult at all to write a ClassLoader that returns a valid class that implements a given interface for any name you ask from it; a List of the classes that implement that interface would then be infinite in length.
In practice, the most common case is an URLClassLoader that looks for classes in a list of filesystem directories and JAR files. So what you need is to get the URLClassLoader, then iterate through those directories and archives, and for each class file you find in them, request the corresponding Class object and look through the return of its getInterfaces() method.

Obviously, Class.isAssignableFrom() tells you whether an individual class implements the given interface. So then the problem is getting the list of classes to test.
As far as I'm aware, there's no direct way from Java to ask the class loader for "the list of classes that you could potentially load". So you'll have to do this yourself by iterating through the visible jars, calling Class.forName() to load the class, then testing it.
However, it's a little easier if you just want to know classes implementing the given interface from those that have actually been loaded:
via the Java Instrumentation framework, you can call Instrumentation.getAllLoadedClasses()
via reflection, you can query the ClassLoader.classes field of a given ClassLoader.
If you use the instrumentation technique, then (as explained in the link) what happens is that your "agent" class is called essentially when the JVM starts up, and passed an Instrumentation object. At that point, you probably want to "save it for later" in a static field, and then have your main application code call it later on to get the list of loaded classes.

If you were asking from the perspective of working this out with a running program then you need to look to the java.lang.* package. If you get a Class object, you can use the isAssignableFrom method to check if it is an interface of another Class.
There isn't a simple built in way of searching for these, tools like Eclipse build an index of this information.
If you don't have a specific list of Class objects to test you can look to the ClassLoader object, use the getPackages() method and build your own package hierarchy iterator.
Just a warning though that these methods and classes can be quite slow.