We are going to Write our Own Custom ClassLoaders in Java, but before we start on this topic.Let’s take a while to understand Why do We need Custom ClassLoaders?
Why Custom ClassLoaders:-
As you know from my previous post JVM has its own ClassLoaders, but this classloader’s loads files from a local file system or a hard drive location. Here are Some of the Use Cases Where we need ClassLoader’s
- What if, Some of the Classes in your application is getting used for a finite period of time. How do we do Unloading of classes after a finite period of time? .This way it helps us for better memory management.
- Load classes from anywhere Classes can be loaded from anywhere, for example, Database, Networks, or even define it on the fly.
Steps Writing Custom ClassLoaders:-
- you need to extend your Custom ClassLoader from java.lang.ClassLoader.
- Override the loadClass() method of ClassLoader, this method is responsible for loading our class.
- If the Class is already loaded it returns that class
- If the class that is requested to load is not present it delegates to parent class loader.
- If Parent class loader is not able load the class, then loadClass() calls findClass() to find and load class
- The class to be loaded must have a public constructor with no arguments.
Custom ClassLoader Example:-
1.Project Structure :-
2. Create a Simple Class:- We create a simple class Frugalis.java, this is the class we load here using our custom ClassLoader.
Frugalis.Java
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package test.frugalis; public class Frugalis { public void printMyName(){ System.out.println("Welcome Frugalis Minds"); } } |
3. Custom ClassLoader :- We create Our custom ClassLoader which extends ClassLoader and override loadClass() .
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package test.main; import java.io.DataInputStream; import java.io.File; import java.io.IOException; import java.io.InputStream; public class MyClassLoader extends ClassLoader { public MyClassLoader(ClassLoader parent) { super(parent); } @Override public Class<?> loadClass(String name) throws ClassNotFoundException { System.out.println("Class Loading Started for " + name); if (name.startsWith("test")) { return getClass(name); } return super.loadClass(name); } /** * Loading of class from .class file * happens here You Can modify logic of * this method to load Class * from Network or any other source * @param name * @return * @throws ClassNotFoundException */ private Class<?> getClass(String name) throws ClassNotFoundException { System.out.println("*********Inside getClass*********"); String file = name.replace('.', File.separatorChar) + ".class"; System.out.println("Name of File" + file); byte[] byteArr = null; try { // This loads the byte code data from the file byteArr = loadClassData(file); System.out.println("Size of byte array "+byteArr.length); Class<?> c = defineClass(name, byteArr, 0, byteArr.length); resolveClass(c); return c; } catch (IOException e) { e.printStackTrace(); return null; } } /** * Loads a given file and converts * it into a Byte Array * @param name * @return * @throws IOException */ private byte[] loadClassData(String name) throws IOException { System.out.println("<<<<<<<<<Inside loadClassData>>>>>>"); InputStream stream = getClass().getClassLoader().getResourceAsStream( name); int size = stream.available(); byte buff[] = new byte[size]; DataInputStream in = new DataInputStream(stream); // Reading the binary data in.readFully(buff); in.close(); return buff; } } |
4. Running the Example:-
We create a Test.java and create an instance of our MyclassLoader, we load a binary class named as test.frugalis.Frugalis and invoke a method printMyName .
Test.java
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package test.main; import java.lang.reflect.InvocationTargetException; public class Test { public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, NoSuchMethodException, Exception { MyClassLoader loader = new MyClassLoader( Test.class.getClassLoader()); System.out.println("loader name---- " +loader.getParent().getClass().getName()); //This Loads the Class we must always //provide binary name of the class Class<?> clazz = loader.loadClass("test.frugalis.Frugalis"); System.out.println("Loaded class name: " + clazz.getName()); //Create instance Of the Class and invoke the particular method Object instance = clazz.newInstance(); clazz.getMethod("printMyName").invoke(instance); } } |
Output:-
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loader name---- sun.misc.Launcher$AppClassLoader Class Loading Started for test.frugalis.Frugalis *********Inside getClass********* Name of Filetest\frugalis\Frugalis.class <<<<<<<<<Inside loadClassData>>>>>> Size of byte array 519 Class Loading Started for java.lang.Object Loaded class name: test.frugalis.Frugalis Class Loading Started for java.lang.System Class Loading Started for java.io.PrintStream Welcome Frugalis Minds |
5. Download Code :-
This was an example of simple example of Custom ClassLoader. Please add your comments and suggestions .
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