Class loader basic

[object Object],1. JVM Architecture

[object Object],[object Object],[object Object],[object Object],2. How Classes are Loaded ?

3. Class Class in Java ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

4. Class Loading ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

5. Class Loaders in JVM ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

6. Class Loaders Rules ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

6. Visibility ,[object Object]

6. Visibility 실습 ,[object Object],[object Object],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 ,[object Object],[object Object],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 ,[object Object],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 ,[object Object]

7. Delegation Model 실습 ,[object Object],[object Object],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 ,[object Object],[object Object],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 ,[object Object],[object Object]

8. Standard Class Loader ,[object Object],[object Object]

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("|"+l+" " +u); } 8. Standard Class Loader ,[object Object],[object Object],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()); } }

[object Object],[object Object],[object Object],8. Standard Class Loader ,[object Object]

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Summary ,[object Object],[object Object],ClassLoader-A Class-X ClassLoader-B ClassLoader-C Class-Y Class-Z Class-Z

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Database Remote Server File system ClassLoader-A Class-X ClassLoader-B ClassLoader-C Class-Y Class-Z Class-R Class-D Summary ,[object Object],[object Object]

[object Object],[object Object],[object Object],[object Object],[object Object],Bootstrap ClassLoader Extensions ClassLoader JVM Application Shared Lib System ClassLoader ,[object Object],[object Object],[object Object],[object Object],[object Object],JAR Summary ,[object Object],[object Object]

[object Object],[object Object],[object Object],[object Object],[object Object],JVM Class Loader A MyClass Class Loader B Class Loader C System ClassLoader Bootstrap ClassLoader Extension ClassLoader Summary ,[object Object],[object Object]

1. Java 내부의 ClassLoader ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

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 ,[object Object],[object Object]

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 ,[object Object],[object Object]

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 ,[object Object],[object Object]

2. Why write a ClassLoader ? ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

3. Class Loader 확장 ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],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 ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],java -classpath ./bin -Dloader.dir=./bin/ FileSystemClassLoader

4. Class Loader 실습 ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],java -classpath ./bin CCLRun Foo aaa bbb

4. Class Loader 실습 ,[object Object],[object Object],java -classpath ./bin athena.loader.PointClient

4. Class Loader 실습 ,[object Object],[object Object],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

1. Class Not Found ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],EJB JAR STRUTS.JAR WAR B AppBForm WAR A AppAForm

[object Object],[object Object],[object Object],[object Object],2. Class Cast ,[object Object],EJB JAR public void utilityMethod (Object o) { ((Apple)o).doSomething(); } WAR B Apple WAR A Apple Apple CLASSCASTEXCEPTION

[object Object],[object Object],[object Object],[object Object],C:gt;java -Xbootclasspath:C:ebug;j2sdk1.4.2_03reibt.jar … [all other boot.class.path jars] app.java 3. Getting Diagnostic Info. ,[object Object]

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(); } } } ,[object Object],3. 실습 ,[object Object]

package demo; public class Rabbit { } Rabbit.java rabbit.jar C:gt; jar tf rabbit.jar demo/ demo.Rabbit.class C:gt; javac –classpath rabbit.jar TopHat.java C:gt; java –classpath ./bin TopHat Command Line Compilation ,[object Object],3. 실습 ,[object Object]

[object Object],[object Object],[object Object],[object Object],Actual Behavior C:gt;java TopHat Exception in thread "main" java.lang.NoClassDefFoundError: demo/Rabbit at TopHat.main(TopHat.java:23) Desired Behavior ,[object Object],3. 실습 ,[object Object]

// 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()); ,[object Object],3. 실습 ,[object Object]

C:gt;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) ,[object Object],3. 실습 ,[object Object]

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(); } } ,[object Object],3. 실습 ,[object Object]

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); } ,[object Object],3. 실습 ,[object Object]

---> 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) ,[object Object],3. 실습 ,[object Object],java -Xbootclasspath/p:D:lassloadertilin -classpath ./bin TopHat

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],3. 실습 ,[object Object]

ClassLoader loader = Thread.currentThread().getContextClassLoader(); loader.loadClass(“demo.Rabbit”).newInstance(); ClassLoader loader = Thread.currentThread().getContextClassLoader(); Class.forName(“demo.Rabbit”,true, loader).newInstance(); – or – ,[object Object],3. 실습 ,[object Object]

---> [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] ,[object Object],3. 실습 ,[object Object]

Class loader basic

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