Let's talk about java class loader

Let's talk about java class loader

• 1.
  Let’s talk about Java ClassLoader Yongqiang
• 2.
  Previously… (about 3years ago)  Let’s talk about JNI  Let’s talk about Java Class File
• 4.
  What’s ClassLoader?  Aclass loader is an object that is responsible for loading classes.  Without classloader, no class can be loaded and run in JVM.  Classes are loaded into JVM and by demand (lazy initialization)  Each ClassLoader has a parent, except the bootstrap one.
• 5.
  Parent-Child Structure of ClassLoader •LoadJava Core libraries (jre/lib/rt.jar) •Implemented by C++ •not derived from java.lang.ClassLoader •No parent Bootstrap Class Loader •Load Java Extension (jre/ext/*.jar) Extensions Class Loader •Load libraries defined in CLASSPATH System Class Loader •Custom class loader derived from java.lang.ClassLoader Custom Class Loader
• 6.
  Bootstrap class loader Extensions class loader Systemclass loader Custom class loader1 Custom class loader2 Default Loaders Custom class loader3
• 8.
  APIs on ClassLoader •Get parent class loader.getParent() • Load the class by given name, return a class instance.loadClass(String name) • Find the class by given name, return a class instancefindClass(String name) • Find the loaded class by given name findLoadedClass(String name) • Generate class instance from its binary representation • Final function defineClass(Strign, byte[] b, int off, int len) • Link given classresolveClass(Class<?> c)
• 9.
  Function of loadClass()
• 10.
  Function of findClass(in URLClassLoader)
• 11.
  URLClassLoader Sample org.yli.test.jar
• 13.
  Q: If urlClassLoader3loads class A first, will the output be changed?
• 14.
  More…  “Jar Hell” PackageA Package B org.yli.test 0.1 org.yli.test 0.2 Package C Q: Any Solution?
• 15.
   OSGi ClassLoader
• 16.
  References  Java ClassLoaderon Wiki  Execution on Java SE specification