Class loading in java

JVM maintains a runtime pool is permgen area where classes are loaded. Whenever a class is referenced defualt class loader finds the class is the class path and loads it into this pool. And this is not specific to user defined classes or classes provided in JDK. When a class id referenced it is loaded into the memory.

Classes loaded by the ClassLoader is stored internally in the ClassLoader instance

// The classes loaded by this class loader. The only purpose of this table
// is to keep the classes from being GC'ed until the loader is GC'ed.
private final Vector<Class<?>> classes = new Vector<>();

When class needs to be added to the memory follwing function is called

// Invoked by the VM to record every loaded class with this loader.
void addClass(Class c) {
    classes.addElement(c);
}

Found an useful diagram on how Class Loaders work.

Classloader hierarchy

Whenever a new JVM is started the bootstrap classloader is responsible to load key Java classes (from java.lang package) and other runtime classes to the memory first. The bootstrap classloader is a parent of all other classloaders. Consequently, it is the only one without a parent.

Next comes the extension classloader. It has the bootstrap classloader as parent and is responsible for loading classes from all .jar files kept in the java.ext.dirs path–these are available regardless of the JVM’s classpath.

The third and most important classloader from a developer’s perspective is the system classpath classloader, which is an immediate child of the extension classloader. It loads classes from directories and jar files specified by the CLASSPATH environment variable, java.class.path system property or -classpath command line option.

ClassLoader Namespace

In Java a class is uniquely identified using ClassLoader + Class as the same class may be loaded by two different class loaders.

Class A loaded by ClassLoader A != Class A loaded by ClassLoader B

How is it helpful?

It is helpful for defining different protection and access policies for different classloaders. Take an example of applet which is loaded using a different classloader, you would not want a third party application all access to your resources. So for security its important to maintain different namespaces.

The Java Virtual Machine starts up by creating an initial class, which is specified in an implementation-dependent manner, using the bootstrap class loader (§5.3.1). The Java Virtual Machine then links the initial class, initializes it, and invokes the public class method void main(String[]). The invocation of this method drives all further execution. Execution of the Java Virtual Machine instructions constituting the main method may cause linking (and consequently creation) of additional classes and interfaces, as well as invocation of additional methods.

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Java Classloader:The Java Classloader is a part of the JRE that dynamically loads Java classes into the JVM. Usually classes are only loaded on demand. The Java run time system does not need to know about files and file systems because of classloaders.

Each Java class must be loaded by a class loader

When the JVM is started, three class loaders are used:

(1) Bootstrap class loader:It loads the core Java libraries located in the <JAVA_HOME>/jre/lib directory. Bootstrap class loade part of the core JVM and written in native code.

(2) Extensions class loader:It loads the code in the extensions directories (<JAVA_HOME>/jre/lib/ext, or any other directory specified by the java.ext.dirs system property)

(3)System class loader:It loads code found on java.class.path, which maps to the CLASSPATH environment variable.

Principles :Java ClassLoader works in three principles delegation, visibility and uniqueness. Delegation is an important concept to understand when learning about classloaders.

Read more: http://javarevisited.blogspot.com/2012/12/how-classloader-works-in-java.html#ixzz2oei79ovs

Flow of Classloader :