Can anyone point me a good resource or explain me about the concept behind Class Loaders? I found the following resource on class loaders http://www.onjava.com/lpt/a/5586 but still no help. The following questions may look silly but trying to answer them always confuses me.
- Why do developers write Custom class loaders, why not invoke a Bootstrap class loader to invoke your custom classes? What is the need to define custom class loaders?
Why there are so many varieties of class loaders? eg: Bootsrap, Comman, Catalina class loader etc.,
Thanks in advance.
I found the following, valid reasons to create custom classloaders:
You want to load a class from an unconventional source (For example, the bytecode for one class is stored in a database, across the network or carried as 0 and 1s by pidgeons - MessengerPidgeonClassLoader). There is some ClassLoader implementations already in the API for such cases, such as URLClassLoader.
You need to define a different hierarchy to load classes. Default implementations of the ClassLoader delegate the search first to the parent, then they try to load the class themselves. Maybe you want a different hierarchy. This is the reason why OSGI and Eclipse have its own ClassLoaders as the Manifest .MF files define all types of weird hierarchy paths (buddy-classloading, for example). All Eclipse classloaders implement the BundleClassLoader interface and have some extra code to find resources within Eclipse Plugins.
You need to do some modification to the bytecode. Maybe the bytecode is encrypted, and you will unencrypt it on the fly (Not that it helps, really, but has been tried). Maybe you want to "patch" the classes loaded on the fly (A la JDO bytecode enhancement).
Using a different classloader than the System Classloader is required if you need to unload classes from memory, or to load classes than could change their definition at runtime. A typical case is an application that generates a class on the fly from an XML file, for example, and then tries to reload this class. Once a class is in the System Classloader, there is no way to unload it and have a new definition.
A common use of classloaders is to isolate a JAR. If you have an application which uses plugins (Eclipse, Maven 2), then you can have this situation: Plugin X needs jar A with version 1.0 while plugin Y need the same jar but version 2.0. X does not run with version 2.0, though.
If you have classloaders, you can create partitions of classes (think of isolated islands connected by thin bridges; the bridges are the classloaders). This way, the classloaders can control what each plugin can see.
When plugin X instantiates a class Foo, which has static fields, this is no problem and there won't be a mixup with the "same" class in plugin Y because each classloader will in fact create its own instance of the class Foo. You then have two classes in memory, where cl1.getName().equals(cl2.getName()) is true but cl1.equals(cl2) is not. This means that instances of cl1 are not assignment compatible to instances of cl2. This can lead to strange ClassCastExceptions which say that org.project.Foo can't be assigned to org.project.Foo.
Just like with remote islands, the two classes are not aware that the other one exists. Think of human clones which are born and then raised on different islands. From the point of view of the VM, there is no problem because instances of the type Class are handled like any other object: There can be several of them. That you think that some of them are "the same" doesn't matter to the VM.
Another use for this pattern is that you can get rid of classes loaded this way: Just make sure that no one has a pointer to any object created from classes loaded from a classloader and then forget about the classloader, too. On the next run of the GC, all classes loaded by this classloader are removed from memory. This allows you to "reload" your application without having to restart the whole VM.
A couple blogs I've written in the deep past about using post-delegation classloaders:
You can't go past the raw source, in cases like this. If you really want the inside dope, the hard core, read the relevant bits of the Java Virtual Machine Specification.
Another good link for java class loaders - Java classloaders
It's extremely rare that you need to create your own ClassLoader. And genereally if you need to, you should already have a really good understanding of what the ClassLoader does.
In other words, if you're asking why you would need to create your own ClassLoader, then you don't need to create one ;)
That being said, I've also seen a ClassLoader being created for an application that dealt with cryptography. This way every time you create a java.netSocket or some kind of file/stream object, instead of using the JVM versions it would use their own special custom built classes. This way they could guarantee the that all information was encrypted and that there were no developer errors.
But it's not very common. You can go a whole Java career without ever needing to create your own custom ClassLoader. Actually if you need to create one, you should really ask if it's necessary.
Why do developers write Custom class loaders, why not invoke a Bootstrap class loader to invoke your custom classes? What is the need to define custom class loaders?
Depending on the application, the developers might override or completely replace the class loading mechanism to suit their needs.
For instance, I have used one application whose classes are loaded from a LDAP :S
Other apps need independent class managing ( like most of the application servers that support hot-deploy )
About resources, there are TONS, in the web, that simply cannot be listed.
An example:
Tomcat use customized WebAppClassloader to load and isolate classes/jars from different web applications.
来源：https://stackoverflow.com/questions/540150/java-class-loaders