Classloader in short

java

Classloader is an abstract class which is responsible for loading classes at runtime. There are 3 known loaders. You can also implement custom loaders to do dynamic loading from anywhere like from network, database, etc.

3 loaders are list below, each is the parent of the lower one.

  • Bootstrap Classloader. It loads classes from JDK under JRE lib folder.
  • Extension Classloader. It loads classes lies in JRE ext folder.
  • App Classloader(aka System Classloader). It loads classes defined in current classpath.

Classloader uses a delegation model. Each Classloader loads a class by delegation to its parent. If the parent cannot load the class, it will load it itself.

object Test {

  class Util

  def main(args: Array[String]): Unit = {
    println(ClassLoader.getSystemClassLoader) // AppClassloader
    println(classOf[Util].getClassLoader) // AppClassloader
    println(classOf[String].getClassLoader) // null
  }
}

Let’s look over the above code.

  1. System classloader is just App classloader, so the first line prints out App loader.
  2. Util is a new class which lies in classpath, so it should be loaded by App loader.
  3. The first two lines returns the same App loader.
  4. String is a type defined in JDK. So it’s loaded by Bootstrap Classloader. And Bootstrap which is implemented by cpp in JVM bootstraps the whole Java app . There’s no corresponding Java class, so it returns null.
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