Class loader basic

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Class Loader
Custom Class Loader
Diagnosing

ClassLoader
1. JVM Architecture

ClassLoader
Class loading takes place in two phases :
Loading
Linking or resolution
2. How Classes are Loaded ?

3. Class Class in Java
ClassLoader
Instances of class java.lang.Class
 represent classes and interfaces in a running Java application
Class has no public constructors
Any applications cannot instantiate it
Class is automatically instantiated by JVM/class loader, when a class loading process is done
Each instance of Class keeps a reference to a class loader that created it

4. Class Loading
ClassLoader
When you define a class, say Foo, and initiate it ( to create its instance)
 i.e. when you execute new Foo();
A local JVM (class loader) tries to
Obtain the class definition of Foo
Given the name of a class (i.e. Foo), the class loader locates a binary representation of Foo’s class definition
Typically, it transforms the class name into a file name, and reads a class file of that name (e.g. Foo.class) from the local file system
If this step fails, JVM return a ClassDefNotFound exception
Parse the obtained class definition into internal structures in the method area of the JVM
The method area is a logical area of memory in the JVM that is used to store information about loaded classes/interfaces
Create an instance of Class, which is associated with Foo
These 3 activities are collectively called class loading.
After successful class loading, an instance of Foo is created and returned

5. Class Loaders in JVM
ClassLoader
Every JVM has two types of class loaders
Default class loaders
Bootstrap class loader
Used to find and load a set of standard class libraries (i.e. java.* classes) that are located in $JAVA_HOME$/jre/lib
-> e.g. $JAVA_HOME$/jre/lib/rt.jar
Extension class loader
Used to find and load a set of extension class libraries (i.e. javax.* classes) that are located in $JAVA_HOME$/jre/lib/ext
System class loader (java.lang.Classloader)
Used to find and load user-defined classes
Custom class loaders
If a user wants to change the behavior of system class loader, the user can extend java.lang.Classloader as a Java class. The class is called a custom class loader
A custom class loader overloads the methods defined in java.lang.Classloader to implement its own class loading behavior.
Different custom class loaders can specialize different class loading behaviors (policies)

6. Class Loaders Rules
ClassLoader
Consistency Rule
Class loaders never load the same class more than once
Delegation Rule
Class loaders always consult a parent class loader before loading a class
Visibility Rule
Classes can only “see” other classes loaded by their class loader’s delegation, the recursive set of a class’s loader and all its parent loaders

6. Visibility
ClassLoader

6. Visibility 실습
ClassLoader
Java Statics
public class Dummy { public static int staticCount = 0; public Dummy(){ staticCount++; System.out.println("Instance #" + staticCount + " constructed."); } } public class StaticTest { public static void main(String[] args) { new Dummy(); new Dummy(); new Dummy(); new Dummy(); new Dummy(); } } Instance #1 constructed. Instance #2 constructed. Instance #3 constructed. Instance #4 constructed. Instance #5 constructed.

6. Visibility 실습
ClassLoader
Java Statics
public class Dummy { public static int staticCount = 0; public Dummy(){ staticCount++; System.out.println("Instance #" + staticCount + " constructed."); } } public class StaticTestClassLoader { public static void main(String[] args) throws Exception { URL[] url = {new File("../target/bin").toURL()}; URLClassLoader cl1 = new URLClassLoader(url); URLClassLoader cl2 = new URLClassLoader(url); URLClassLoader cl3 = new URLClassLoader(url); cl1.loadClass("Dummy").newInstance(); cl2.loadClass("Dummy").newInstance(); cl3.loadClass("Dummy").newInstance(); } } Instance #1 constructed. Instance #1 constructed. Instance #1 constructed.

7. Delegation Model
ClassLoader
class NetworkClassLoader extends ClassLoader { String host; int port; public Class findClass(String name) { byte[] b = loadClassData(name); return defineClass(name, b, 0, b.length); } private byte[] loadClassData(String name) { // load the class data from the connection . . . } }

7. Delegation Model
ClassLoader

7. Delegation Model 실습
ClassLoader
Java 1.1 Style
public class TestClassLoader extends ClassLoader { protected Hashtable m_classes = new Hashtable(); protected boolean m_cacle = false; protected byte[] loadClassBytes(String name) throws ClassNotFoundException{ byte[] result = null; try{ FileInputStream in = new FileInputStream("../Target/bin/" + name.replace('.', '/') + ".class"); ByteArrayOutputStream data = new ByteArrayOutputStream(); int ch; while((ch = in.read()) != -1) data.write(ch); result = data.toByteArray(); } catch (Exception e){ throw new ClassNotFoundException(name); } return result; } protected synchronized Class loadClass (String name, boolean resolve) throws ClassNotFoundException{ System.out.println(" 요청된 Class : " + name); Class result = (Class)m_classes.get(name); try{ if(result==null) return super.findSystemClass(name); else return result; } catch(Exception e) {} System.out.println("TestClassLoader Loading Class : " + name); byte bytes[] = loadClassBytes(name); result = defineClass(name, bytes, 0, bytes.length); if(result == null) throw new ClassNotFoundException(name); if(resolve) resolveClass(result); m_classes.put(name, result); return result; } }

7. Delegation Model 실습
ClassLoader
Java 1.2 Style
public class TestClassLoader extends ClassLoader { protected byte[] loadClassBytes(String name) throws ClassNotFoundException{ byte[] result = null; try{ FileInputStream in = new FileInputStream("../Target/bin/" + name.replace('.', '/') + ".class"); ByteArrayOutputStream data = new ByteArrayOutputStream(); int ch; while((ch = in.read()) != -1) data.write(ch); result = data.toByteArray(); } catch (Exception e){ throw new ClassNotFoundException(name); } return result; } protected Class findClass(String name) throws ClassNotFoundException{ System.out.println(" 요청된 Class : " + name); System.out.println("TestClassLoader Loading Class : " + name); byte bytes[] = loadClassBytes(name); return defineClass(name, bytes, 0, bytes.length); } }

//java.lanag.ClassLoader 일부 protected synchronized Class loadClass(String name, boolean resolve) throws ClassNotFoundException { // First, check if the class has already been loaded Class c = findLoadedClass(name); if (c == null) { try { if (parent != null) { c = parent.loadClass(name, false); } else { c = findBootstrapClass0(name); } } catch (ClassNotFoundException e) { // If still not found, then invoke findClass in order // to find the class. c = findClass(name); } } if (resolve) { resolveClass(c); } return c; } 7. Delegation Model 실습
ClassLoader
Java 1.2 Style

8. Standard Class Loader
ClassLoader
Standard ClassLoader

ClassLoader
Java Options

public class ClassLoaderTree { public static void printClassLoaderTree(ClassLoader l) { if(l== null){ System.out.println("BootStrap ClassLoader"); return; } ClassLoader p = l.getParent(); if ( p!= null) { printClassLoaderTree(p); } else { System.out.println("BootStrap ClassLoader"); } String u = ""; if ( l instanceof URLClassLoader) { u = getURLs( ((URLClassLoader)l).getURLs()); } // end of if () System.out.println("t|n"+l+" " +u); } 8. Standard Class Loader
ClassLoader
ClassLoader Tree
public static String getURLs(URL[] urls) { if ( urls == null) { return "{}"; } // end of if () StringBuffer b = new StringBuffer("{"); for ( int i = 0;i<urls.length;i++) { b.append( urls[i] ).append(":"); } // end of for () b.append("}"); return b.toString(); } public static void main(String[] args) { ClassLoaderTree.printClassLoaderTree (ClassLoaderTree.class.getClassLoader()); } }

java -classpath ./bin ClassLoaderTree
java -Djava.ext.dirs=./bin ClassLoaderTree
java -Xbootclasspath:.bin;D:JDKSDK13jrelibrt.jar;... ClassLoaderTree
ClassLoader

What is a class loader?
mechanism for loading classes into the JVM
subclass of java.lang.ClassLoader
Hierarchical structure
Parent-child relationships
Uses delegation
Load requests delegated to parent
Summary
ClassLoader
ClassLoader Basics
ClassLoader-A Class-X ClassLoader-B ClassLoader-C Class-Y Class-Z Class-Z

Unidirectional visibility
Parent-loaded classes cannot see child-loaded classes
Thread.getContextClassloader()
Unique Namespaces
Classes referenced as classloader.classname
Variety of class file locations
File system
Databases
Remote server locations
Database Remote Server File system ClassLoader-A Class-X ClassLoader-B ClassLoader-C Class-Y Class-Z Class-R Class-D Summary
ClassLoader
ClassLoader Basics

Bootstrap ClassLoader
aka Primordial ClassLoader
Created by the JVM
Loads the JDK classes included in the JVM
(including java.lang.ClassLoader )
Bootstrap ClassLoader Extensions ClassLoader JVM Application Shared Lib System ClassLoader
System ClassLoader
loads classes specified in the classpath
Loads applications, shared libraries, jars, etc.
Extensions ClassLoader
Loads JRE classes
JAR Summary
ClassLoader
ClassLoader Basics

Determine Initial ClassLoader
Delegate to Parent ClassLoader
Class Loaded?
No – Delegate to Parent
Lastly – If class is not loaded in parent hierarchy then the Initial loader will attempt to load the class itself.
JVM Class Loader A MyClass Class Loader B Class Loader C System ClassLoader Bootstrap ClassLoader Extension ClassLoader Summary
ClassLoader
Typical Loading Scenario

Class Loader
Custom Class Loader
Diagnosing

1. Java 내부의 ClassLoader
Custom ClassLoader
java.security.SecureClassLoader
Java 2 Platform 보안부분 지원
Java.net.URLClassLoader
Java 2 Runtime Library 내부에서 유일하게 실제 구현된 ClassLoader
sun.applet.AppletClassLoader
java.rmi.server.RMIClassLoader
RMI runtime system 에서 class 를 마샬링하고 로딩하는 기능을 하는 wrapper class
sun.rmi.server.LoaderHandler Class 에 대한 다리 역할
BootStrap ClassLoader
Native Code 로 구현
Sun.boot.class.path 프로퍼티 참조
sun.misc.Launcher$ExtClassLoader
Java.ext.dirs 프로퍼티 참조
sun.misc.Launcher$AppClassLoader

public static void main(String[] args) throws Exception { URL[] urlArray = { //new java.io.File("subdir.jar").toURL() // For loading-from-subdir.jar, uncomment the // above line and comment out the below line new java.io.File("subdir/").toURL() // For loading-from-subdirectory-subdir, // comment out the first line and uncomment // out this one }; URLClassLoader ucl = new URLClassLoader(urlArray); Object obj = ucl.loadClass("Hello").newInstance(); // Hello should print "Hello" to the System.out stream } 1.URLClassLoader 사용 예
ClassLoader
FileURLClient

public static void main(String[] args) throws Exception { URL[] urlArray = { new URL("http", "www.javageeks.com", "/SSJ/examples/") }; URLClassLoader ucl = new URLClassLoader(urlArray); Object obj = ucl.loadClass("com.javageeks.util.Hello").newInstance(); // Hello should print "Hello from JavaGeeks.com!" to the // System.out stream } 1.URLClassLoader 사용 예
ClassLoader
HttpURLClient

public static void main(String[] args) throws Exception { URL[] urlArray = { new URL("ftp", "reader:password@www.javageeks.com", "/examples") }; URLClassLoader ucl = new URLClassLoader(urlArray); Object obj = ucl.loadClass("Hello").newInstance(); } 1.URLClassLoader 사용 예
ClassLoader
FTPURLClient

2. Why write a ClassLoader ?
Custom ClassLoader
The default ClassLoader only knows how to load class files from the local file system.
대부분의 경우에 만족
일반적으로 Local File System, Network 으로 부터 File 을 Loading 하는 것 이외에 다음과 같은 용도로 사용됨
Automatically verify a digital signature before executing untrusted code
Transparently decrypt code with a user-defined password
Create dynamically built classes customized to the user’s specific needs

3. Class Loader 확장
Custom ClassLoader
findClass(…)
Class 이름으로 부터 Byte Code 를 얻는데 사용
defineClass(…) 를 호출하여 Class Object 로 변환함
findResource(…)
주어진 임의의 이름에 대한 Byte Code 를 얻는데 사용
getResource(..) 에서 호출함
findLibrary(…)
ClassLoader 가 native library 를 포함한 Class 를 Load 할 때 호출
Binary Data 가 아닌 절대 경로명 만 반환
protected Class findClass(String name) throws ClassNotFoundException { throw new ClassNotFoundException(name); } protected URL findResource(String name) { return null; } protected String findLibrary(String libname) { return null; }

4. Class Loader 실습
Custom ClassLoader
FileSystemClassLoader
Local File System 에서 Class Byte Code 를 뽑아내는 ClassLoader 제작
.jar/.zip 에 대한 부분은 고려하지 않음
ClassLoader Class 를 extends 하여 구현
findClass(String name) 부분 overriding 구현
java -classpath ./bin -Dloader.dir=./bin/ FileSystemClassLoader

Custom ClassLoader
CompilingClassLoader
When a class is requested, see if it exists on the disk, in the current directory, or in the appropriate subdirectory
If the class is not available, but the source is, call the java compiler to generate the class file
If the class file does exist, check to see if it is older than its source code. If it is older than the source, call the java compiler to generate the class file.
If the compile fails, or if for any reason the class file could not be generated from existing source, throw a ClassNotFoundException
If we still don’t have the class, maybe it’s in some other library, so call findSystemClass to see if that will work
If we still don’t have the class, throw a ClassNotFoundException
Otherwise, return the class
java -classpath ./bin CCLRun Foo aaa bbb

Custom ClassLoader
HotDeploy
java -classpath ./bin athena.loader.PointClient

Custom ClassLoader
Context Class Loader
java -classpath ./bin DynamicLoader Echo aaa aaa aaa java -Djava.ext.dirs=./jar -classpath ./bin DynamicLoader Echo aaa aaa aaa java -Djava.ext.dirs=./jar -classpath ./bin FixedDynamicLoader Echo aaa aaa aaa

Class Loader
Custom Class Loader
Diagnosing

1. Class Not Found
Diagnosing
ClassNotFoundException
Used String representation
(e.g. ClassLoader.LoadClass(className))
Typically, dependent Class is only visible to children
NoClassDefFoundError
Class existed at compile time
Typically, dependent Class is only visible to children
NoClassDefFoundException
An interesting phenomenon: this Exception is not in the JDK,
but it appears in dozens of topics in developer forums 
EJB JAR STRUTS.JAR WAR B AppBForm WAR A AppAForm

ClassCastException
same Class was loaded by two different ClassLoaders
ClassCircularityError
A Class and one of its dependents are both dependent on a third Class; different ClassLoaders are used to load that third Class
2. Class Cast
Diagnosing
EJB JAR public void utilityMethod (Object o) { ((Apple)o).doSomething(); } WAR B Apple WAR A Apple Apple CLASSCASTEXCEPTION

Place diagnostic statements in the application code
Place diagnostic statements in a custom ClassLoader
Place diagnostic statements in the core Java ClassLoaders. The -Xbootclasspath option makes to possible to supersede core Classes in a development environment.
Use the –verbose option
C:>java -Xbootclasspath:C:debug;j2sdk1.4.2_03jrelibrt.jar … [all other boot.class.path jars] app.java 3. Getting Diagnostic Info.
Diagnosing

import java.io.File; import java.net.MalformedURLException; import java.net.URLClassLoader; import java.net.URL; import demo.Rabbit; /** * Instantiates and displays a Rabbit. */ public class TopHat { public static void main (String args[]) { try { // Create a ClassLoader that knows where to find demo. Rabbit URL rabbitURL = new File("rabbit.jar").toURL(); URL[] urls = new URL[]{rabbitURL}; URLClassLoader rabbitClassLoader = new URLClassLoader(urls,Thread.currentThread().getContextClassLoader()); // Set the ContextClassLoader for the current Thread so that it can find Rabbit.class Thread.currentThread().setContextClassLoader(rabbitClassLoader); // Make a Rabbit appear. System.out.println(new Rabbit()); } catch (MalformedURLException malformedURLException) { malformedURLException.printStackTrace(); } } }
TopHat.java
3. 실습
Diagnosing

package demo; public class Rabbit { } Rabbit.java rabbit.jar C:> jar tf rabbit.jar demo/ demo.Rabbit.class C:> javac –classpath rabbit.jar TopHat.java C:> java –classpath ./bin TopHat Command Line Compilation
Rabbit.java
3. 실습
Diagnosing

TopHat should instantiate and display a String
representation of a Rabbit, even though
rabbit.jar is not referenced on the application
classpath.
Actual Behavior C:>java TopHat Exception in thread "main" java.lang.NoClassDefFoundError: demo/Rabbit at TopHat.main(TopHat.java:23) Desired Behavior
Desired Behavior Vs. Actual Behavior
3. 실습
Diagnosing

// Create a ClassLoader that knows where to find demo. Rabbit URL rabbitURL = new File("rabbit.jar").toURL(); URL[] urls = new URL[]{rabbitURL}; URLClassLoader rabbitClassLoader = new URLClassLoader(urls,Thread.currentThread().getContextClassLoader()); System.out.println("---> ClassLoader for TopHat: " + TopHat.class.getClassLoader()); System.out.println("---> Before setting a custom ContextClassLoader, ContextClassLoader is: “); System.out.println( Thread.currentThread().getContextClassLoader()); // Set the ContextClassLoader for the current Thread so that it can find // Rabbit Thread.currentThread().setContextClassLoader(rabbitClassLoader); System.out.println("---> After setting a custom ContextClassLoader ContextClassLoader is: "); System.out.println( Thread.currentThread().getContextClassLoader()); // Make a Rabbit appear. System.out.println(new Rabbit());
TopHat.java with Diagnostic Statements
3. 실습
Diagnosing

C:>java TopHat ---> ClassLoader for TopHat: sun.misc.Launcher$AppClassLoader@e80a59 ---> Before setting a custom ContextClassLoader, ContextClassLoader is: sun.misc.Launcher$AppClassLoader@e80a59 ---> After setting a custom ContextClassLoader, ContextClassLoader is: [email_address] Exception in thread "main" java.lang.NoClassDefFoundError: demo/Rabbit at TopHat.main(TopHat.java:28)
Output From Diagnostics in the Application
3. 실습
Diagnosing

protected Class findClass(final String name) throws ClassNotFoundException { try { return (Class) AccessController.doPrivileged(new PrivilegedExceptionAction() { public Object run() throws ClassNotFoundException { String path = name.replace('.', '/').concat(".class"); Resource res = ucp.getResource(path, false); if (res != null) { try { return defineClass(name, res); } catch (IOException e) { throw new ClassNotFoundException(name, e); } } else { System.out.println(“--  Looked in:”); URL[] urls = getURLs(); for (int i = 0;i<urls.length;i++) { System.out.println(urls[i]); } throw new ClassNotFoundException(name); } } }, acc); } catch (java.security.PrivilegedActionException pae) { throw (ClassNotFoundException) pae.getException(); } }
Diagnostics in java.net.URLClassLoader
3. 실습
Diagnosing

protected synchronized Class loadClass(String name, boolean resolve) throws ClassNotFoundException { Class c = findLoadedClass(name); if (c == null) { try { if (name.equals("demo.Rabbit")) System.out.println("---> " + this + "is asking parent (" + parent + ") to try to load " + name + "."); if (parent != null) { c = parent.loadClass(name, false); } else { c = findBootstrapClass0(name); } } catch (ClassNotFoundException e) { // If still not found, then call findClass in order // to find the class. if (name.equals("demo.Rabbit")) System.out.println("---> " + name + " was not found by parent (" + parent + "), so " + this + " will try."); c = findClass(name); } } if (resolve) { resolveClass(c); } return c; } void addClass(Class c) { if (c.toString().equals("class demo.Rabbit")) System.out.println("---> " + this + "loaded " + c); classes.addElement(c); } private synchronized Class loadClassInternal(String name)throws ClassNotFoundException { if (name.equals("demo.Rabbit")) { System.out.println("JVM is requesting that " + this +" load "+ name); } return loadClass(name); }
Diagnostics in java.lang.ClassLoader
3. 실습
Diagnosing

---> JVM is requesting that sun.misc.Launcher$AppClassLoader@e80a59 load demo.Rabbit ---> sun.misc.Launcher$AppClassLoader@e80a59 is asking parent (sun.misc.Launcher$ ExtClassLoader@1ff5ea7) to try to load demo.Rabbit. ---> sun.misc.Launcher$ExtClassLoader@1ff5ea7 is asking parent (null) to try to load demo.Rabbit. ---> demo.Rabbit was not found by parent (null), so sun.misc.Launcher$ExtClassLoader@1ff5ea7 will try. --> Looked in: file:/C:/j2sdk1.4.2_03/jre/lib/ext/dnsns.jar file:/C:/j2sdk1.4.2_03/jre/lib/ext/ldapsec.jar file:/C:/j2sdk1.4.2_03/jre/lib/ext/localedata.jar file:/C:/j2sdk1.4.2_03/jre/lib/ext/sunjce_provider.jar ---> demo.Rabbit was not found by parent (sun.misc.Launcher$ExtClassLoader@1ff5ea7), so sun.misc.Launcher$AppClassLoader@e80a59 will try. --> Looked in: file:/C:/ Exception in thread "main" java.lang.NoClassDefFoundError: demo/Rabbit at TopHat.main(TopHat.java:28)
Output from Diagnostics in the Core Java Classes
3. 실습
Diagnosing
java -Xbootclasspath/p:D:ClassloaderUtilbin -classpath ./bin TopHat

Regardless of how the Context ClassLoader is set , the classloader that loaded the calling Class will always be used to load a requested Class when no ClassLoader is specified
new ClassName()
Class.forName(“package.ClassName”)
Context ClassLoaders are used by
JNDI
JAXP
Analysis
3. 실습
Diagnosing

ClassLoader loader = Thread.currentThread().getContextClassLoader(); loader.loadClass(“demo.Rabbit”).newInstance(); ClassLoader loader = Thread.currentThread().getContextClassLoader(); Class.forName(“demo.Rabbit”,true, loader).newInstance(); – or –
Possible Solutions
3. 실습
Diagnosing

---> [email_address] is asking parent (sun.misc.Launcher$AppClassL oader@e80a59) to try to load demo.Rabbit. ---> sun.misc.Launcher$AppClassLoader@e80a59 is asking parent (sun.misc.Launcher$ ExtClassLoader@1ff5ea7) to try to load demo.Rabbit. ---> sun.misc.Launcher$ExtClassLoader@1ff5ea7 is asking parent (null) to try to load demo.Rabbit. ---> demo.Rabbit was not found by parent (null), so sun.misc.Launcher$ExtClassLo ader@1ff5ea7 will try. --> Looked in: file:/C:/j2sdk1.4.2_03/jre/lib/ext/dnsns.jar file:/C:/j2sdk1.4.2_03/jre/lib/ext/ldapsec.jar file:/C:/j2sdk1.4.2_03/jre/lib/ext/localedata.jar file:/C:/j2sdk1.4.2_03/jre/lib/ext/sunjce_provider.jar ---> demo.Rabbit was not found by parent (sun.misc.Launcher$ExtClassLoader@1ff5e a7), so sun.misc.Launcher$AppClassLoader@e80a59 will try. --> Looked in: file:/C:/ ---> demo.Rabbit was not found by parent (sun.misc.Launcher$AppClassLoader@e80a5 9), so java.net.URLClassLoader@defa1a will try. ---> [email_address] loaded class demo.Rabbit [email_address]
Rabbit Appears
3. 실습
Diagnosing

Class loader basic

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