Everything you need to know about Java Classloaders [Spring IO Barcelona 2015]

Classloaders: a New Hope.

In the session, we’ll get into a wonderful world where we control exactly where from and what classes does the JVM load. We will design a powerful plugin system that carefully balances flexibility and simplicity. We’ll define classes from bytes, from urls, even from a photo of a rabbit.

But the path to mastery is not a walk in the park. We’ll find mean exception, then even meaner exceptions. Then exceptions whose message doesn’t fit on the screen and is undecipherable. We’ll have to debug everything, challenge assumptions and show that your will is stronger than the soulless stubborness of Tomcat’s classloading scheme.

This session covers the basic functionality and principles of work of Java Classloaders and goes through a series of real world examples to teach how to treat the common issues that custom classloading can create.

Transcript

1. Oleg Šelajev
   @shelajev
   ZeroTurnaround
   Everything
   you need to know about
   Java Classloaders
   

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2. whoami
   @shelajev
   

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4. • Basics
   • Practice
   • Problems
   

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5. What is common?
   

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6. A class loader is an object that is responsible
   for loading classes.
   Classloader
   

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7. Separate namespaces for applications
   Each WAR module of EAR gets a classloader
   Class loaders have a parent class loader
   Java EE web module local classes are searched first
   Intentions
   

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8. Separate namespaces for applications
   Each WAR module of EAR gets a classloader
   Class loaders have a parent class loader
   Java EE web module local classes are searched first
   Intentions
   

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9. Separate namespaces for applications
   Each WAR module of EAR gets a classloader
   Build a hierarchy of classloaders
   Java EE web module local classes are searched first
   Intentions
   

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10. public class A {
    void doSmth() {
    B b = new B();
    b.doSmthElse();
    }
    }
    Trivial case
    

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11. public class A {
    void doSmth() {
    B b = new B();
    b.doSmthElse();
    }
    }
    Trivial case
    A.class.getClassLoader().loadClass("B");
    

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12. public abstract class Classloader {
    public Class findClass(String name);
    public Class loadClass(String name);
    protected Class defineClass(byte[] b);
    public URL getResource(String name);
    public Enumeration getResources(String name);
    public ClassLoader getParent();
    }
    java.lang.Classloader
    

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13. public abstract class Classloader {
    public Class findClass(String name);
    public Class loadClass(String name);
    protected Class defineClass(byte[] b);
    public URL getResource(String name);
    public Enumeration getResources(String name);
    public ClassLoader getParent();
    }
    java.lang.Classloader
    

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14. public abstract class Classloader {
    public Class findClass(String name);
    public Class loadClass(String name);
    protected Class defineClass(byte[] b);
    public URL getResource(String name);
    public Enumeration getResources(String name);
    public ClassLoader getParent();
    }
    java.lang.Classloader
    

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15. public abstract class Classloader {
    public Class findClass(String name);
    public Class loadClass(String name);
    protected Class defineClass(byte[] b);
    public URL getResource(String name);
    public Enumeration getResources(String name);
    public ClassLoader getParent();
    }
    java.lang.Classloader
    

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16. • Basics
    • Practice
    • Problems
    

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17. Plugin Manager
    

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18. • Basics
    • Practice
    • Problems
    

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19. ClassNotFoundException
    NoClassDefFoundError
    Not enough classes
    

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20. IDE class lookup
    find *.jar -exec jar -tf '{}' \; | grep MyClass
    URLClassLoader.getUrls()
    Container specific logs
    

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21. IncompatibleClassChangeError
    AbstractMethodError
    NoSuch(Method|Field)FoundError
    ClassCastException, IllegalAccessError
    Different classes
    

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22. -verbose:class
    ClassLoader.getResource()
    javap -private MyClass
    

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23. LinkageError
    ClassCastException, IllegalAccessError
    Too many classes
    

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24. -verbose:class
    ClassLoader.getResource()
    

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25. Each JAR has own class loader
    Class loaders are siblings & central repository
    Explicit imports / exports
    Repository can find relevant class loaders by
    package
    Modern patterns
    

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26. Validate assumptions
    Don’t write more classloaders :)
    JREBEL rocks!
    Takeaways
    

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27. oleg@zeroturnaround.com
    @shelajev
    github.com/zeroturnaround/jf-hw-classloaders
    Contact me
    

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28. http://0t.ee/jrebel-springio
    

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29. Classloader leaks
    

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30. Recreate the object
    Old classloader New classloader
    MyObject.class
    MyObject
    MyObject.class
    MyObject
    

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