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  2247  次浏览      31
 

Ò»¡¢Ê²Ã´ÊÇClassLoader?

´ó¼Ò¶¼ÖªµÀ£¬µ±ÎÒÃÇдºÃÒ»¸öJava³ÌÐòÖ®ºó£¬²»ÊǹÜÊÇCS»¹ÊÇBSÓ¦Ó㬶¼ÊÇÓÉÈô¸É¸ö.classÎļþ×éÖ¯¶ø³ÉµÄÒ»¸öÍêÕûµÄJavaÓ¦ÓóÌÐò£¬µ±³ÌÐòÔÚÔËÐÐʱ£¬¼´»áµ÷ÓøóÌÐòµÄÒ»¸öÈë¿Úº¯ÊýÀ´µ÷ÓÃϵͳµÄÏà¹Ø¹¦ÄÜ£¬¶øÕâЩ¹¦Äܶ¼±»·â×°ÔÚ²»Í¬µÄclassÎļþµ±ÖУ¬ËùÒÔ¾­³£Òª´ÓÕâ¸öclassÎļþÖÐÒªµ÷ÓÃÁíÍâÒ»¸öclassÎļþÖеķ½·¨£¬Èç¹ûÁíÍâÒ»¸öÎļþ²»´æÔڵģ¬Ôò»áÒý·¢ÏµÍ³Òì³£¡£¶ø³ÌÐòÔÚÆô¶¯µÄʱºò£¬²¢²»»áÒ»´ÎÐÔ¼ÓÔØ³ÌÐòËùÒªÓõÄËùÓÐclassÎļþ£¬¶øÊǸù¾Ý³ÌÐòµÄÐèÒª£¬Í¨¹ýJavaµÄÀà¼ÓÔØ»úÖÆ£¨ClassLoader£©À´¶¯Ì¬¼ÓÔØÄ³¸öclassÎļþµ½ÄÚ´æµ±Öе쬴ӶøÖ»ÓÐclassÎļþ±»ÔØÈëµ½ÁËÄÚ´æÖ®ºó£¬²ÅÄܱ»ÆäËüclassËùÒýÓá£ËùÒÔClassLoader¾ÍÊÇÓÃÀ´¶¯Ì¬¼ÓÔØclassÎļþµ½ÄÚ´æµ±ÖÐÓõġ£

¶þ¡¢JavaĬÈÏÌṩµÄÈý¸öClassLoader

BootStrap ClassLoader£º³ÆÎªÆô¶¯Àà¼ÓÔØÆ÷£¬ÊÇJavaÀà¼ÓÔØ²ã´ÎÖÐ×î¶¥²ãµÄÀà¼ÓÔØÆ÷£¬¸ºÔð¼ÓÔØJDKÖеĺËÐÄÀà¿â£¬È磺rt.jar¡¢resources.jar¡¢charsets.jarµÈ£¬¿Éͨ¹ýÈçϳÌÐò»ñµÃ¸ÃÀà¼ÓÔØÆ÷´ÓÄÄЩµØ·½¼ÓÔØÁËÏà¹ØµÄjar»òclassÎļþ£º

URL[] urls =
sun.misc.Launcher.getBootstrapClassPath().getURLs();
for (int i = 0; i < urls.length; i++) {
System.out.println(urls[i].toExternalForm());
}

ÒÔÏÂÄÚÈÝÊÇÉÏÊö³ÌÐò´Ó±¾»úJDK»·¾³Ëù»ñµÃµÄ½á¹û£º

file: /C:/Program%20Files/ Java/jdk1.6.0_22/jre/lib/resources.jar
file: /C:/Program%20Files/Java/jdk1.6.0_22/jre/lib/rt.jar
file: /C:/Program%20Files/Java/jdk1.6.0_22/jre/lib/sunrsasign.jar
file: /C:/Program%20Files/Java/jdk1.6.0_22/jre/lib/jsse.jar
file: /C:/Program%20Files/Java/jdk1.6.0_22/jre/lib/jce.jar
file: /C:/Program%20Files/Java/jdk1.6.0_22/jre/lib/charsets.jar
file: /C:/Program%20Files/Java/jdk1.6.0_22/jre/classes/

ÆäʵÉÏÊö½á¹ûÒ²ÊÇͨ¹ý²éÕÒsun.boot.class.pathÕâ¸öϵͳÊôÐÔËùµÃÖªµÄ¡£

System.out.println(System.getProperty
("sun.boot.class.path"));

´òÓ¡½á¹û£º

C:\Program Files\Java\jdk1.6.0_22\jre\lib\resources.jar;
C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar;
C:\Program Files\Java\jdk1.6.0_22\jre\lib\sunrsasign.
<>jar;C:\Program Files\Java\jdk1.6.0_22\jre\lib\jsse.jar;
C:\Program Files\Java\jdk1.6.0_22\jre\lib\jce.jar;
C:\Program Files\Java\jdk1.6.0_22\jre\lib\charsets.jar;
C:\Program Files\Java\jdk1.6.0_22\jre\classes

Extension ClassLoader£º³ÆÎªÀ©Õ¹Àà¼ÓÔØÆ÷£¬¸ºÔð¼ÓÔØJavaµÄÀ©Õ¹Àà¿â£¬Ä¬ÈϼÓÔØJAVA_HOME/jre/lib/ext/ĿϵÄËùÓÐjar¡£

App ClassLoader£º³ÆÎªÏµÍ³Àà¼ÓÔØÆ÷£¬¸ºÔð¼ÓÔØÓ¦ÓóÌÐòclasspathĿ¼ÏµÄËùÓÐjarºÍclassÎļþ¡£

×¢Ò⣺ ³ýÁËJavaĬÈÏÌṩµÄÈý¸öClassLoaderÖ®Í⣬Óû§»¹¿ÉÒÔ¸ù¾ÝÐèÒª¶¨Òå×ÔÒѵÄClassLoader£¬¶øÕâЩ×Ô¶¨ÒåµÄClassLoader¶¼±ØÐë¼Ì³Ð×Ôjava.lang.ClassLoaderÀ࣬Ҳ°üÀ¨JavaÌṩµÄÁíÍâ¶þ¸öClassLoader£¨Extension ClassLoaderºÍApp ClassLoader£©ÔÚÄÚ£¬µ«ÊÇBootstrap ClassLoader²»¼Ì³Ð×ÔClassLoader£¬ÒòΪËü²»ÊÇÒ»¸öÆÕͨµÄJavaÀ࣬µ×²ãÓÉC++±àд£¬ÒÑǶÈëµ½ÁËJVMÄں˵±ÖУ¬µ±JVMÆô¶¯ºó£¬Bootstrap ClassLoaderÒ²Ëæ×ÅÆô¶¯£¬¸ºÔð¼ÓÔØÍêºËÐÄÀà¿âºó£¬²¢¹¹ÔìExtension ClassLoaderºÍApp ClassLoaderÀà¼ÓÔØÆ÷¡£

Èý¡¢ClassLoader¼ÓÔØÀàµÄÔ­Àí

1¡¢Ô­Àí½éÉÜ

ClassLoaderʹÓõÄÊÇË«Ç×ίÍÐÄ£ÐÍÀ´ËÑË÷ÀàµÄ£¬Ã¿¸öClassLoaderʵÀý¶¼ÓÐÒ»¸ö¸¸Àà¼ÓÔØÆ÷µÄÒýÓ㨲»ÊǼ̳еĹØÏµ£¬ÊÇÒ»¸ö°üº¬µÄ¹ØÏµ£©£¬ÐéÄâ»úÄÚÖõÄÀà¼ÓÔØÆ÷£¨Bootstrap ClassLoader£©±¾ÉíûÓи¸Àà¼ÓÔØÆ÷£¬µ«¿ÉÒÔÓÃ×÷ÆäËüClassLoaderʵÀýµÄµÄ¸¸Àà¼ÓÔØÆ÷¡£µ±Ò»¸öClassLoaderʵÀýÐèÒª¼ÓÔØÄ³¸öÀàʱ£¬Ëü»áÊÔͼÇ××ÔËÑË÷ij¸öÀà֮ǰ£¬ÏȰÑÕâ¸öÈÎÎñίÍиøËüµÄ¸¸Àà¼ÓÔØÆ÷£¬Õâ¸ö¹ý³ÌÊÇÓÉÉÏÖÁÏÂÒÀ´Î¼ì²éµÄ£¬Ê×ÏÈÓÉ×î¶¥²ãµÄÀà¼ÓÔØÆ÷Bootstrap ClassLoaderÊÔͼ¼ÓÔØ£¬Èç¹ûû¼ÓÔØµ½£¬Ôò°ÑÈÎÎñת½»¸øExtension ClassLoaderÊÔͼ¼ÓÔØ£¬Èç¹ûҲû¼ÓÔØµ½£¬Ôòת½»¸øApp ClassLoader ½øÐмÓÔØ£¬Èç¹ûËüҲûÓмÓÔØµÃµ½µÄ»°£¬Ôò·µ»Ø¸øÎ¯Íеķ¢ÆðÕߣ¬ÓÉËüµ½Ö¸¶¨µÄÎļþϵͳ»òÍøÂçµÈURLÖмÓÔØ¸ÃÀà¡£Èç¹ûËüÃǶ¼Ã»ÓмÓÔØµ½Õâ¸öÀàʱ£¬ÔòÅ׳öClassNotFoundExceptionÒì³£¡£·ñÔò½«Õâ¸öÕÒµ½µÄÀàÉú³ÉÒ»¸öÀàµÄ¶¨Ò壬²¢½«Ëü¼ÓÔØµ½ÄÚ´æµ±ÖУ¬×îºó·µ»ØÕâ¸öÀàÔÚÄÚ´æÖеÄClassʵÀý¶ÔÏó¡£

2¡¢ÎªÊ²Ã´ÒªÊ¹ÓÃË«Ç×ίÍÐÕâÖÖÄ£ÐÍÄØ£¿

ÒòΪÕâÑù¿ÉÒÔ±ÜÃâÖØ¸´¼ÓÔØ£¬µ±¸¸Ç×ÒѾ­¼ÓÔØÁ˸ÃÀàµÄʱºò£¬¾ÍûÓбØÒª×ÓClassLoaderÔÙ¼ÓÔØÒ»´Î¡£¿¼Âǵ½°²È«ÒòËØ£¬ÎÒÃÇÊÔÏëһϣ¬Èç¹û²»Ê¹ÓÃÕâÖÖίÍÐģʽ£¬ÄÇÎÒÃǾͿÉÒÔËæÊ±Ê¹ÓÃ×Ô¶¨ÒåµÄStringÀ´¶¯Ì¬Ìæ´újavaºËÐÄapiÖж¨ÒåµÄÀàÐÍ£¬ÕâÑù»á´æÔڷdz£´óµÄ°²È«Òþ»¼£¬¶øË«Ç×ίÍеķ½Ê½£¬¾Í¿ÉÒÔ±ÜÃâÕâÖÖÇé¿ö£¬ÒòΪStringÒѾ­ÔÚÆô¶¯Ê±¾Í±»Òýµ¼Àà¼ÓÔØÆ÷£¨Bootstrcp ClassLoader£©¼ÓÔØ£¬ËùÒÔÓû§×Ô¶¨ÒåµÄClassLoaderÓÀÔ¶Ò²ÎÞ·¨¼ÓÔØÒ»¸ö×Ô¼ºÐ´µÄString£¬³ý·ÇÄã¸Ä±äJDKÖÐClassLoaderËÑË÷ÀàµÄĬÈÏËã·¨¡£

3¡¢ µ«ÊÇJVMÔÚËÑË÷ÀàµÄʱºò£¬ÓÖÊÇÈçºÎÅж¨Á½¸öclassÊÇÏàͬµÄÄØ£¿

JVMÔÚÅж¨Á½¸öclassÊÇ·ñÏàͬʱ£¬²»½öÒªÅжÏÁ½¸öÀàÃûÊÇ·ñÏàͬ£¬¶øÇÒÒªÅжÏÊÇ·ñÓÉͬһ¸öÀà¼ÓÔØÆ÷ʵÀý¼ÓÔØµÄ¡£Ö»ÓÐÁ½ÕßͬʱÂú×ãµÄÇé¿öÏ£¬JVM²ÅÈÏΪÕâÁ½¸öclassÊÇÏàͬµÄ¡£¾ÍËãÁ½¸öclassÊÇͬһ·Ýclass×Ö½ÚÂ룬Èç¹û±»Á½¸ö²»Í¬µÄClassLoaderʵÀýËù¼ÓÔØ£¬JVMÒ²»áÈÏΪËüÃÇÊÇÁ½¸ö²»Í¬class¡£±ÈÈçÍøÂçÉϵÄÒ»¸öJavaÀàorg.classloader.simple.NetClassLoaderSimple£¬javac±àÒëÖ®ºóÉú³É×Ö½ÚÂëÎļþNetClassLoaderSimple.class£¬ClassLoaderAºÍClassLoaderBÕâÁ½¸öÀà¼ÓÔØÆ÷²¢¶ÁÈ¡ÁËNetClassLoaderSimple.classÎļþ£¬²¢·Ö±ð¶¨Òå³öÁËjava.lang.ClassʵÀýÀ´±íʾÕâ¸öÀ࣬¶ÔÓÚJVMÀ´Ëµ£¬ËüÃÇÊÇÁ½¸ö²»Í¬µÄʵÀý¶ÔÏ󣬵«ËüÃÇȷʵÊÇͬһ·Ý×Ö½ÚÂëÎļþ£¬Èç¹ûÊÔͼ½«Õâ¸öClassʵÀýÉú³É¾ßÌåµÄ¶ÔÏó½øÐÐת»»Ê±£¬¾Í»áÅ×ÔËÐÐʱÒì³£java.lang.ClassCaseException£¬ÌáʾÕâÊÇÁ½¸ö²»Í¬µÄÀàÐÍ¡£ÏÖÔÚͨ¹ýʵÀýÀ´ÑéÖ¤ÉÏÊöËùÃèÊöµÄÊÇ·ñÕýÈ·£º

1£©¡¢ÔÚweb·þÎñÆ÷ÉϽ¨Ò»¸öorg.classloader.simple.NetClassLoaderSimple.javaÀà

package org.classloader.simple;

public class NetClassLoaderSimple {

private NetClassLoaderSimple instance;

public void setNetClassLoaderSimple(Object obj) {
this.instance = (NetClassLoaderSimple)obj;
}
}
org.classloader.simple.NetClassLoaderSimpleÀàµÄsetNetClassLoaderSimple·½·¨½ÓÊÕÒ»¸öObjectÀàÐͲÎÊý£¬²¢½«ËüÇ¿ÖÆ×ª»»³Éorg.classloader.simple.NetClassLoaderSimpleÀàÐÍ¡£

2£©¡¢²âÊÔÁ½¸öclassÊÇ·ñÏàͬ£¨NetWorkClassLoader.java£©

package classloader;

public class NewworkClassLoaderTest {

public static void main(String[] args) {
try {
//²âÊÔ¼ÓÔØÍøÂçÖеÄclassÎļþ
String rootUrl = "http://localhost:8080/httpweb/classes";
String className = "org.classloader.simple.NetClassLoaderSimple";
NetworkClassLoader ncl1 = new NetworkClassLoader(rootUrl);
NetworkClassLoader ncl2 = new NetworkClassLoader(rootUrl);
Class<?> clazz1 = ncl1.loadClass(className);
Class<?> clazz2 = ncl2.loadClass(className);
Object obj1 = clazz1.newInstance();
Object obj2 = clazz2.newInstance();
clazz1.getMethod("setNetClassLoaderSimple", Object.class).invoke(obj1, obj2);
} catch (Exception e) {
e.printStackTrace();
}
}

}

Ê×ÏÈ»ñµÃÍøÂçÉÏÒ»¸öclassÎļþµÄ¶þ½øÖÆÃû³Æ£¬È»ºóͨ¹ý×Ô¶¨ÒåµÄÀà¼ÓÔØÆ÷NetworkClassLoader´´½¨Á½¸öʵÀý£¬²¢¸ù¾ÝÍøÂçµØÖ··Ö±ð¼ÓÔØÕâ·Ýclass£¬²¢µÃµ½ÕâÁ½¸öClassLoaderʵÀý¼ÓÔØºóÉú³ÉµÄClassʵÀýclazz1ºÍclazz2£¬×îºó½«ÕâÁ½¸öClassʵÀý·Ö±ðÉú³É¾ßÌåµÄʵÀý¶ÔÏóobj1ºÍobj2£¬ÔÙͨ¹ý·´Éäµ÷ÓÃclazz1ÖеÄsetNetClassLoaderSimple·½·¨¡£

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½áÂÛ£º´Ó½á¹ûÖпÉÒÔ¿´³ö£¬ËäÈ»ÊÇͬһ·Ýclass×Ö½ÚÂëÎļþ£¬µ«ÊÇÓÉÓÚ±»Á½¸ö²»Í¬µÄClassLoaderʵÀýËù¼ÓÔØ£¬ËùÒÔJVMÈÏΪËüÃǾÍÊÇÁ½¸ö²»Í¬µÄÀà¡£

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²âÊÔ1£º´òÓ¡ClassLoaderÀàµÄ²ã´Î½á¹¹£¬Çë¿´ÏÂÃæÕâ¶Î´úÂ룺

ClassLoader loader =
ClassLoaderTest.class.getClassLoader();
//»ñµÃ¼ÓÔØClassLoaderTest.classÕâ¸öÀàµÄÀà¼ÓÔØÆ÷
while(loader != null) {
System.out.println(loader);
loader = loader.getParent(); //»ñµÃ¸¸Àà¼ÓÔØÆ÷µÄÒýÓÃ
}
System.out.println(loader);

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µÚ¶þÐнá¹û˵Ã÷£ºAppClassLoaderµÄÀà¼ÓÆ÷ÊÇExtClassLoader£¬¼´parent=ExtClassLoader¡£

µÚÈýÐнá¹û˵Ã÷£ºExtClassLoaderµÄÀà¼ÓÆ÷ÊÇBootstrap ClassLoader£¬ÒòΪBootstrap ClassLoader²»ÊÇÒ»¸öÆÕͨµÄJavaÀ࣬ËùÒÔExtClassLoaderµÄparent=null£¬ËùÒÔµÚÈýÐеĴòÓ¡½á¹ûΪnull¾ÍÊÇÕâ¸öÔ­Òò¡£

²âÊÔ2£º½«ClassLoaderTest.class´ò°ü³ÉClassLoaderTest.jar£¬·Åµ½Extension ClassLoaderµÄ¼ÓÔØÄ¿Â¼Ï£¨JAVA_HOME/jre/lib/ext£©£¬È»ºóÖØÐÂÔËÐÐÕâ¸ö³ÌÐò£¬µÃµ½µÄ½á¹û»áÊÇʲôÑùÄØ£¿

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[Loaded java.io.FileReader from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded sun.nio.cs.StreamDecoder from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.util.ArrayList from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.lang.reflect.Array from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.util.Locale from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.util .concurrent.ConcurrentMap from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.util .concurrent.ConcurrentHashMap from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.util .concurrent.locks.Lock from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.util .concurrent.locks.ReentrantLock from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.util .concurrent.ConcurrentHashMap$Segment from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.util .concurrent.locks.AbstractOwnableSynchronizer from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.util .concurrent.locks.AbstractQueuedSynchronizer from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.util .concurrent.locks.ReentrantLock$Sync from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.util .concurrent.locks.ReentrantLock$NonfairSync from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.util .concurrent.locks.AbstractQueuedSynchronizer$Node from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.util .concurrent.ConcurrentHashMap$HashEntry from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.lang .CharacterDataLatin1 from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.io. ObjectStreamClass from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded sun.net. www.ParseUtil from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.util.BitSet from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.net.Parts from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.net.URLStreamHandler from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded sun.net.www.protocol.file.Handler from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.util.HashSet from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded sun.net.www.protocol.jar.Handler from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded sun.misc.Launcher$AppClassLoader from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded sun.misc.Launcher$AppClassLoader$1 from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.lang .SystemClassLoaderAction from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Path C:\Program Files\Java\jdk1.6.0_22\jre\classes]
[Loaded classloader. ClassLoaderTest from C:\Program Files\Java\jdk1.6.0_22\jre\classes]
null //ÕâÊÇ´òÓ¡µÄ½á¹û
C:\Program Files\Java\jdk1.6.0_22\jre\lib\resources.jar;
C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar;
C:\Program Files\Java\jdk1.6.0_22\jre\lib\sunrsasign.jar; C:\Program Files\Java\jdk1.6.0_22\jre\lib\jsse.jar;
C:\Program Files\Java\jdk1.6.0_22\jre\lib\jce.jar; C:\Program Files\Java\jdk1.6.0_22\jre\lib\charsets.jar;
C:\Program Files\Java\jdk1.6.0_22\jre\classes; c:\ClassLoaderTest.jar
//ÕâÒ»¶ÎÊÇSystem.out.println (System.getProperty("sun.boot.class.path")); ´òÓ¡³öÀ´µÄ¡£Õâ¸ö·¾¶¾ÍÊÇBootstrcp ClassLoaderĬÈÏËÑË÷ÀàµÄ·¾¶
[Loaded java.lang.Shutdown from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]
[Loaded java.lang.Shutdown$Lock from C:\Program Files\Java\jdk1.6.0_22\jre\lib\rt.jar]

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´Ó½á¹ûÖпÉÒÔ¿´³ö£¬Á½ÖÖ·½Ê½¶¼ÊµÏÖÁ˽«ClassLoaderTest.classÓÉBootstrcp ClassLoader¼ÓÔØ³É¹¦ÁË¡£

ËÄ¡¢¶¨Òå×ÔÒѵÄClassLoader

¼ÈÈ»JVMÒѾ­ÌṩÁËĬÈϵÄÀà¼ÓÔØÆ÷£¬ÎªÊ²Ã´»¹Òª¶¨Òå×ÔÒѵÄÀà¼ÓÔØÆ÷ÄØ£¿

ÒòΪJavaÖÐÌṩµÄĬÈÏClassLoader£¬Ö»¼ÓÔØÖ¸¶¨Ä¿Â¼ÏµÄjarºÍclass£¬Èç¹ûÎÒÃÇÏë¼ÓÔØÆäËüλÖõÄÀà»òjarʱ£¬±ÈÈ磺ÎÒÒª¼ÓÔØÍøÂçÉϵÄÒ»¸öclassÎļþ£¬Í¨¹ý¶¯Ì¬¼ÓÔØµ½ÄÚ´æÖ®ºó£¬Òªµ÷ÓÃÕâ¸öÀàÖеķ½·¨ÊµÏÖÎÒµÄÒµÎñÂß¼­¡£ÔÚÕâÑùµÄÇé¿öÏ£¬Ä¬ÈϵÄClassLoader¾Í²»ÄÜÂú×ãÎÒÃǵÄÐèÇóÁË£¬ËùÒÔÐèÒª¶¨Òå×Ô¼ºµÄClassLoader¡£

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1¡¢¼Ì³Ðjava.lang.ClassLoader

2¡¢ÖØÐ´¸¸ÀàµÄfindClass·½·¨

¶ÁÕß¿ÉÄÜÔÚÕâÀïÓÐÒÉÎÊ£¬¸¸ÀàÓÐÄÇô¶à·½·¨£¬ÎªÊ²Ã´Æ«Æ«Ö»ÖØÐ´findClass·½·¨£¿

ÒòΪJDKÒѾ­ÔÚloadClass·½·¨ÖаïÎÒÃÇʵÏÖÁËClassLoaderËÑË÷ÀàµÄËã·¨£¬µ±ÔÚloadClass·½·¨ÖÐËÑË÷²»µ½Ààʱ£¬loadClass·½·¨¾Í»áµ÷ÓÃfindClass·½·¨À´ËÑË÷À࣬ËùÒÔÎÒÃÇÖ»ÐèÖØÐ´¸Ã·½·¨¼´¿É¡£ÈçûÓÐÌØÊâµÄÒªÇó£¬Ò»°ã²»½¨ÒéÖØÐ´loadClassËÑË÷ÀàµÄËã·¨¡£ÏÂͼÊÇAPIÖÐClassLoaderµÄloadClass·½·¨£º

ʾÀý£º×Ô¶¨ÒåÒ»¸öNetworkClassLoader£¬ÓÃÓÚ¼ÓÔØÍøÂçÉϵÄclassÎļþ

package classloader;

import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import java.net.URL;

/**
* ¼ÓÔØÍøÂçclassµÄClassLoader
*/
public class NetworkClassLoader extends ClassLoader {

private String rootUrl;

public NetworkClassLoader(String rootUrl) {
this.rootUrl = rootUrl;
}

@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
Class clazz = null;//this.findLoadedClass(name); // ¸¸ÀàÒѼÓÔØ
//if (clazz == null) { //¼ì²é¸ÃÀàÊÇ·ñÒѱ»¼ÓÔØ¹ý
byte[] classData = getClassData(name); //¸ù¾ÝÀàµÄ¶þ½øÖÆÃû³Æ,»ñµÃ¸ÃclassÎļþµÄ×Ö½ÚÂëÊý×é
if (classData == null) {
throw new ClassNotFoundException();
}
clazz = defineClass(name, classData, 0, classData.length); //½«classµÄ×Ö½ÚÂëÊý×éת»»³ÉClassÀàµÄʵÀý
//}
return clazz;
}

private byte[] getClassData(String name) {
InputStream is = null;
try {
String path = classNameToPath(name);
URL url = new URL(path);
byte[] buff = new byte[1024*4];
int len = -1;
is = url.openStream();
ByteArrayOutputStream baos = new ByteArrayOutputStream();
while((len = is.read(buff)) != -1) {
baos.write(buff,0,len);
}
return baos.toByteArray();
} catch (Exception e) {
e.printStackTrace();
} finally {
if (is != null) {
try {
is.close();
} catch(IOException e) {
e.printStackTrace();
}
}
}
return null;
}

private String classNameToPath(String name) {
return rootUrl + "/" + name.replace(".", "/") + ".class";
}

}

²âÊÔÀࣺ

package classloader;

public class ClassLoaderTest {

public static void main(String[] args) {
try {
/*ClassLoader loader = ClassLoaderTest.class.getClassLoader(); //»ñµÃClassLoaderTestÕâ¸öÀàµÄÀà¼ÓÔØÆ÷
while(loader != null) {
System.out.println(loader);
loader = loader.getParent(); //»ñµÃ¸¸¼ÓÔØÆ÷µÄÒýÓÃ
}
System.out.println(loader);*/

String rootUrl = "http://localhost:8080/httpweb/classes";
NetworkClassLoader networkClassLoader = new NetworkClassLoader(rootUrl);
String classname = "org.classloader.simple.NetClassLoaderTest";
Class clazz = networkClassLoader.loadClass(classname);
System.out.println(clazz.getClassLoader());

} catch (Exception e) {
e.printStackTrace();
}
}

}

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ÏÂͼÊÇÎÒ»úÆ÷ÉÏweb·þÎñÆ÷µÄĿ¼½á¹¹£º

Ŀǰ³£ÓÃweb·þÎñÆ÷Öж¼¶¨ÒåÁË×Ô¼ºµÄÀà¼ÓÔØÆ÷£¬ÓÃÓÚ¼ÓÔØwebÓ¦ÓÃÖ¸¶¨Ä¿Â¼ÏµÄÀà¿â£¨jar»òclass£©£¬È磺Weblogic¡¢Jboss¡¢tomcatµÈ£¬ÏÂÃæÎÒÒÔTomcatΪÀý£¬Õ¹Ê¾¸ÃwebÈÝÆ÷¶¼¶¨ÒåÁËÄÄЩ¸öÀà¼ÓÔØÆ÷£º

1¡¢Ð½¨Ò»¸öweb¹¤³Ìhttpweb

2¡¢Ð½¨Ò»¸öClassLoaderServletTest£¬ÓÃÓÚ´òÓ¡webÈÝÆ÷ÖеÄClassLoader²ã´Î½á¹¹

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ClassLoaderServletTest extends HttpServlet {

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

response.setContentType("text/html");
PrintWriter out = response.getWriter();
ClassLoader loader = this.getClass().getClassLoader();
while(loader != null) {
out.write(loader.getClass().getName()+"<br/>");
loader = loader.getParent();
}
out.write(String.valueOf(loader));
out.flush();
out.close();
}

public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
this.doGet(request, response);
}

}

3¡¢ÅäÖÃServlet£¬²¢Æô¶¯·þÎñ

4¡¢·ÃÎÊServlet£¬»ñµÃÏÔʾ½á¹û

   
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