What is a Java ClassLoader?
The Java Classloader is a part of the JRE (Java Runtime Environment) that dynamically loads Java classes into the JVM (Java Virtual Machine). In particular, a Java program, unlike one written in C or C++, isn't a single executable file, but instead is composed of many individual class files, each of which corresponds to a single Java class. Normally classes are only loaded on demand. This means, these Java class files are not loaded into memory all at once, but rather are loaded on demand, as needed by the program (Class Loader). Class Loader is a component with the Java Execution Engine which loads the Binary data from the .class files available in the classpath into the Method Area . Loading of a class into the method area occurs only the first time when the class is referenced with in the running Java application. For all other references the data is reused from the method area, unless the class has been UNLOADED .ClassLoader in Java works on three principle:
- Delegation
- Visibility
- Uniqueness
- Bootstrap class loader
- Extensions class loader
- System class loader
Building a SimpleClassLoader
A class loader starts by being a subclass of java.lang.ClassLoader . The only abstract method that must be implemented is loadClass(). The flow of loadClass() is as follows:- Verify class name.
- Check to see if the class requested has already been loaded.
- Check to see if the class is a "system" class.
- Attempt to fetch the class from this class loader's repository.
- Define the class for the Virtual Machine.
- Resolve the class.
- Return the class to the caller.
How the very first class loaded?
Class loaders are hierarchical. The initial class is loaded with the help of public static main() method declared in your class. All the subsequently loaded classes are loaded by the classes, which are already loaded and running.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. Second phase 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 Java Virtual Machine'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. Most Java programmers will never need to explicitly use class loaders (except to load resources so that it still works when they're bundled in JARs), let alone write their own. ClassLoaders are used in very large systems and server applications to do things like:- Modularize a system and load, unload and update modules at runtime
- Use different versions of an API library (e.g. an XML parser) in parallel
- Isolate different applications running within the same JVM (ensuring they don't interfere with each other, e.g. through static variables)
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