??????????д????????java classloader?????java classloader???????
???????????????飬??????Щ?????????????????????????
??????????Java?????????javac???????????????class????????class????а?????????????????л??????????????Σ??????????Щ????????????????????????????????й??????????????????????????????????????????????????????????????в??????????????class????????й????е????????????????????class??????object??????java???????????????????????????????public??????νУ?class????????У???????????SomeObject.class????????????Class????
????Java???????·?????????????????????java api???????Class???и???????getClassLoader()???????????????????classloader????? ????????????????????????????￡??????????class?????????class loader???????????Class????????????
?????????????????????JAVA_HOME?? CLASSPATH??JAVA_HOME???????java??????·????????????????洦???????java???????????????й???????????JAVA_HOME/bin??·??????PATH?С?????CLASSPATH???????????CLASSPATH???????????jar??tools.jar?? dt.jar?? rt.jar????
????????????tools.jar??????Щjava ???????????openjdk??????jdk???????langtools???·???μ???????????? ?????д?Щ?????????????java???????????java?????????????tools?????javac????api??dt.jar?????swing?????Щ????rt.jar????????????????java???????е?java??
??????????????java?????????????????????JAVA?г???Bootstrap class loader ???????parent class loader??????java ???????“Understanding Extension Class Loading”???????????????????????????????????rt.jar??i18n.jar?????????????????????????????classpath??????????????????????????Bootstrap class loader????????????java???е????class loader??bootstrap class loader???????·???????????jre/lib/ext?????? extension class loader??ExtClassLoader??????. ??????????????????class loader??AppClassLoader??????????????????￡???????java??????????????-classpath ?? -cp??????????????б????????????·????????????б?)???????и??????????class loader?????????sun.misc.Launcher???У? ??????Launcher??constructor?????е?????
public Launcher() {
// Create the extension class loader
ClassLoader extcl;
try {
extcl = ExtClassLoader.getExtClassLoader();
} catch (IOException e) {
throw new InternalError(
"Could not create extension class loader"?? e);
}
// Now create the class loader to use to launch the application
try {
loader = AppClassLoader.getAppClassLoader(extcl);
} catch (IOException e) {
throw new InternalError(
"Could not create application class loader"?? e);
}
// Also set the context class loader for the primordial thread.
Thread.currentThread().setContextClassLoader(loader);
…..
}
?????????????????·???????????????????????????о??
Java Api?е?ClassLoader???е?loadClass??????
protected Class<?> loadClass(String name?? boolean resolve)
throws ClassNotFoundException
{
synchronized (getClassLoadingLock(name)) {
// First?? check if the class has already been loaded
Class c = findLoadedClass(name);
if (c == null) {
long t0 = System.nanoTime();
try {
if (parent != null) {
c = parent.loadClass(name?? false);
} else {
c = findBootstrapClassOrNull(name);
}
} catch (ClassNotFoundException e) {
// ClassNotFoundException thrown if class not found
// from the non-null parent class loader
}
if (c == null) {
// If still not found?? then invoke findClass in order
// to find the class.
long t1 = System.nanoTime();
c = findClass(name);
// this is the defining class loader; record the stats
sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
sun.misc.PerfCounter.getFindClasses().increment();
}
}
if (resolve) {
resolveClass(c);
}
return c;
}
}
???????????????????classloader?????????parent class loader????????????????????м??????????????????????????????????????????????java.lang.String?? ?????loadClass??????????????????parent classloader?в???????????????bootstrap class loader????????????????????????????????????????????????????????????????????????????ε?classloader??????·???μ??????????????????????????????????????С??????????????web container????????tomcat?????jsp??涼?????????????class??????????work·???У????????web??????????????classloader?????????·???е?class?????
????????????????????????jvm??????????У???????????????·????????????jar????????????μ????????????????Σ???????????????檔?????и???????????????????????????????治?У????????????classloader???????????????????????????????classloader????????????????????java???????????????class???????binary name????class metadata?????????????????????????????????????????????????????