java-classloader.md

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The Overview of Java Classloader

In Java, a class loader is a subsystem of the Java Virtual Machine (JVM) that is responsible for loading Java classes at runtime.

Java uses a hierarchical class loading system, which means that each class loader has a parent class loader. When a class is requested, the class loader first checks its own cache of loaded classes. If the class is not found, it delegates the request to its parent class loader. This process continues until the class is found or until the root class loader is reached.

There are three types of class loaders in Java:

1. Bootstrap Class Loader: It is the parent of all class loaders and is responsible for loading the core Java libraries from the rt.jar file.

2. Extension Class Loader: It is the child of the Bootstrap Class Loader and is responsible for loading the classes from the extensions directory.

3. System Class Loader: It is the child of the Extension Class Loader and is responsible for loading classes from the classpath and any other custom class loaders.

In addition to these built-in class loaders, Java also provides a mechanism to create custom class loaders. This can be useful in scenarios where you want to load classes from non-standard locations or protect the application from loading classes that are not trusted.

Overall, understanding class loaders is an important aspect of Java programming, as it enables developers to control how classes are loaded and provides a way to customize the application's behavior.

Java Classloader FAQ

1. What is a classloader in Java?

A classloader is a part of the Java Runtime Environment (JRE) that dynamically loads Java classes into memory as they are needed during runtime.

2. What are the types of classloaders in Java?

There are three types of classloaders in Java: the bootstrap classloader, the extension classloader, and the system classloader.

3. What is the bootstrap classloader?

The bootstrap classloader is responsible for loading the core Java libraries, such as java.lang.* and java.util.*, that are required for the JVM to start up.

4. What is the extension classloader?

The extension classloader is responsible for loading classes from the extension directories, which are specified by the java.ext.dirs system property.

5. What is the system classloader?

The system classloader is responsible for loading classes from the classpath, which is specified by the java.class.path system property.

6. What is the delegation model of classloaders?

The delegation model of classloaders is a mechanism by which classloaders delegate the task of loading a class to their parent classloader before attempting to load the class themselves. This ensures that classes are loaded only once, and that the same class is used across multiple classloaders.

7. What is class loading?

Class loading is the process by which a classloader loads a class into memory.

8. What is the classpath in Java?

The classpath is a system environment variable that specifies the directories or JAR files containing the Java class files that the JVM needs to execute a program.

9. How does the classloader resolve class conflicts?

The classloader resolves class conflicts by using the delegation model. When a class is requested, the classloader first delegates the task of loading the class to its parent classloader. If the parent classloader is unable to find the class, the classloader attempts to load the class itself.

10. Can you create your own classloader in Java?

Yes, you can create your own classloader in Java by extending the java.lang.ClassLoader class and overriding its loadClass() method.