I am studying the jvm class loading process,and got know the following fact:
the Class Loader Subsystem uses a delegation hierarchy algorithm
It will search in order from BootStrap path --> extension path-->Application path.
Then I want to "try" this in real code as below :
create a class A, and Class B; Class A with the main() method and a type B variable.
after compiling , I used jar command to get jar file of Class B from B.class, then delete the B.class file, put the B.jar file to jdk/jre/lib/ext
(Since the oracle doc says the extClassLoader will not search any loose classfile in jdk/jre/lib/ext).
The result is ClassNotFoundException .so My question is :
1) Does jvm not allow loading of user's clasfile from BootStrap and Extension classpath ?
2) if so , why its loading using a delegation hierarchy algorithm which seems to result in low efficiency ?
tks ....
Things seem bit strange.
After several times of ClassNotFoundException , I got java.lang.IllegalAccessError which at least indicates the extclassloader is trying to access my class B.jar
since I did not declare a package for Class A and B, This might be the reason, so I just added a public modifier before my Class B and did the same as above , finally the ExtClassLoader works withs my B.jar.
though still no idea why first times the jvm ignores my jar file and later on read it ....seems jvm keeps a catche of directory list(of BootStrap and Ext class path)which does not update just-in-time like my computer system
It sounds like you are doing something wrong in your deployment.
I have used lib/endorsedin the past without problem even for internal java.lang* classes.
The JVM doesn't cache anything from one run to the next.
I have created a custom Java ClassLoader but I can't get the JVM to load it. I'm getting a ClassNotFoundException when I launch the application. The JAR that contains my code is added to the CLASSPATH as an argument to the JVM. Now I am wondering if I have the JAR in the correct place...do ClassLoaders get added here? I know that application code gets added to the CLASSPATH, but one could argue if a custom ClassLoader is application code or internal to the JVM. So, any ideas?
EDIT:
Ok, after thinking about this more, I think I may asking the JVM to do something circular. The ClassLoader I have created is a system class loader. These class loaders are the ones that search CLASSPATH for classes to load...but I have put the class loader itself there! So, if I'm correct, where do custom class loaders go?
I have read that Inheritance is a "compile-time' phenomenon. Also in a different place I have read that the superclass code is loaded by classloader, which I deduce happens at run-time. This is causing me some confusion regarding the nature of inheritance. Does the class file of sublass contain the actual compiled code of superclass, or is it accessed at run-time?
So consider you create a class that inherits a class that is included in a 3rd party jar file.
In order to compile your code you need to have the 3rd party jar file in the classpath of your compiler.
In order to run your code you will also need the jar file in the classpath of the java command that launches the application.
Your subclass does not contain the code of the superclass, it is in the jar files. Your compiled class contains a reference to the superclass. When your class is loaded by the classloader it searches the classpath for the superclass and loads it.
Where did you read it's compile time? I guess if you're compiling your subclass then yes, it needs to have a superclass to reference when being compiled.
But when you actually run the code it is dynamically linked as per:
http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-5.html
"The Java Virtual Machine dynamically loads, links and initializes classes and interfaces"
So, I'm working in eclipse were everything compiles and runs correctly. However, when compiling under ant for the build server, A large number of tests fail with a NoSuchMethodError saying:
class A implements B
interface B extends C
C requires method getSyncID() // standard getter for an int field.
A.java contains getSyncID()
A.class contains getSyncID()
and yet the Error is still thrown.
Does anyone know how the hell this could happen? how to fix it.
This happens when class files get out of sync with each other; in other words, one was compiled to a new version while another one wasn't. Try cleaning and rebuilding from scratch.
The problem occurs when the class loader discovers that a method in one class (A) calls a method in another class (B) that does not exist. The root cause is that the class B that the class loader sees is different to the class B that the compiler saw when it compiled A.
The most common cause of this problem is as #MarkPeters says - that your ".class" files have gotten out of sync with the ".java" files and need to be recompiled from scratch.
Another possibility is that you have an old copy of some of the ".class" files on the classpath when you try to run the application.
But either way, you have to believe the classloader. If it says that the method is not there, then it is not there ... in the particular ".class" file that it is loading. If the method appears to be there in the ".class" file, then that is evidence that you are loading a different version of the file earlier in the classpath!
I have read a lot about the Java class loading process lately. Often I came across texts that claimed that it is not possible to add classes to the classpath during runtime and load them without class loader hackery (URLClassLoaders etc.)
As far as I know classes are loaded dynamically. That means their bytecode representation is only loaded and transformed to a java.lang.Class object when needed.
So shouldn't it be possible to add a JAR or *.class file to the classpath after the JVM started and load those classes, provided they haven't been loaded yet? (To be clear: In this case the classpath is simply folder on the filesystem. "Adding a JAR or *.class file" simply means dropping them in this folder.)
And if not, does that mean that the classpath is searched on JVM startup and all fully qualified names of the found classes are cached in an internal "list"?
It would be nice of you if you could point me to some sources in your answers. Preferably the offical SUN documentation: Sun JVM Spec. I have read the spec but could not find anything about the classpath and if it's finalized on JVM startup.
P.s.
This is a theoretical question. I just want to know if it is possible. There is nothing practical I want to achieve. There is just my thirst for knowledge :)
There are two concepts here that are being intermixed: The classpath and the class files in the classpath.
If you point the classpath to a directory, you will generally have no issue adding a file to the directory and having it picked up as part of the classpath. Due to the potential size of all classes in the classpath it isn't really feasible for a modern JVM to load them all at startup. However this is of limited value as it will not include Jar files.
However, changing the classpath itself (which directories, jars, etc. are searched) on a running JVM will depend very much on the implementation. As far as I know, on standard Sun JVMs there is no documented (as in guaranteed to work) method of accomplishing this.
In general, if this is something you need to do (have a dynamic classpath that changes at runtime) then you want to be implementing a ClassLoader, if for no other reason than to be able to throw it away and make a new one that doesn't reference those classes anymore if they need to be unloaded.
However, for a small amount of dynamic loading there are better ways. In Java 1.6 you can specify all the jar files in a directory (*.jar) so you can tell users to put additional libraries in a specified location (although they have to be there at startup).
You also have the option of including a jar file or other location in the classpath even though you don't need it, as a placeholder for someone to put an optional jar or resource file there (such as a log configuration file).
But if you need serious dynamic class loading and especially unloading while the application is running, a Classloader implementation is required.
Since nobody could give my a definite answer nor a link to a corresponding part of the documentation I provide a answer myself. Nevertheless I would like to thank everybody that tried to answer the question.
Short answer:
The classpath is not final upon JVM start.
You actually can put classes in the classpath after the JVM started and they will be loaded.
Long answer:
To answer this question I went after user unknowns suggestion and wrote a little test program.
The basic idea is to have two classes. One is the main class which instantiates the second class. On startup the second class is not on the classpath. After the cli program started it'll prompt you to press enter. Before you press enter you copy the second class on the classpath. After you press enter the second class is instantiated. If the classpath would be final on JVM startup this would throw an Exception. But it doesn't. So I assume the classpath is not final on JVM startup.
Here are the source codes:
JVMTest.java
package jvmtest;
import java.io.Console;
import jvmtest.MyClass;
public class JVMTest {
public static void main(String[] args) {
System.out.println("JVMTest started ...");
Console c = System.console();
String enter = c.readLine("Press Enter to proceed");
MyClass myClass = new MyClass();
System.out.println("Bye Bye");
}
}
MyClass.java
package jvmtest;
public class MyClass {
public MyClass() {
System.out.println("MyClass v2");
}
}
The folder structure looks like this:
jvmtest/
JVMTest.class
MyClass.class
I started the cli program with this command:
> java -cp /tmp/ jvmtest.JVMTest
As you can see I had my jvmtest folder in /tmp/jvmtest. You obviously have to change this according to where you put the classes.
So here are the steps I performed:
Make sure only JVMTest.class is in jvmtest.
Start the program with the command from above.
Just to be sure press enter. You should see an Exception telling you that no class could be found.
Now start the program again.
After the program started and you are prompted to press enter copy the MyClass file into the jvmtest folder.
Press enter. You should see "MyClass v1".
Additional notes:
This also worked when I packed the MyClass class in a jar and run the test above.
I ran this on my Macbook Pro running Mac OS X 10.6.3
> Java -version
results in:
java version "1.6.0_20"
Java(TM) SE Runtime Environment (build 1.6.0_20-b02-279-10M3065)
Java HotSpot(TM) 64-Bit Server VM (build 16.3-b01-279, mixed mode)
#Jen I don't think your experiment can prove your theory, because it is more about object instantiation: your printline happens when an object of this class is instantiated, but not necessarily telling that JVM knows your code, the class, just when it is instantiating.
My opinion is that all Java classes are loaded when JVM is up, and it is possible to plug in more classes into JVM while it is running: this technique is called: Hot deployment.
Bottom line: it is possible to add entries to the system classpath at runtime, and is shown how. This, however, has irreversible side-effects and relies on Sun JVM implementation details.
Class path is final, in the most literal sense:
The system class loader (the one that loads from the main class path) is sun.misc.Launcher$AppClassLoader in rt.jar.
rt.jar:sun/misc/Launcher.class (sources are generated with Java Decompiler):
public class Launcher
{
<...>
static class AppClassLoader
extends URLClassLoader
{
final URLClassPath ucp = SharedSecrets.getJavaNetAccess().getURLClassPath(this);
<...>
rt.jar:sun/misc/URLClassLoader.class:
protected Class<?> findClass(final String paramString)
throws ClassNotFoundException
{
<...>
String str = paramString.replace('.', '/').concat(".class");
Resource localResource = URLClassLoader.this.ucp.getResource(str, false);
<...>
But, even if the field is final, this doesn't mean we can't mutate the object itself if we somehow get access to it. The field is without an access modifier - which means, we can access it if only we make the call from the same package. (the following is IPython with JPype; the commands are readable enough to easily derive their Java counterparts)
#jpype doesn't automatically add top-level packages except `java' and `javax'
In [28]: jpype.sun=jpype._jpackage.JPackage("sun")
In [32]: jpype.sun.misc.Launcher
Out[32]: jpype._jclass.sun.misc.Launcher
In [35]: jpype.sun.misc.Launcher.getLauncher().getClassLoader()
Out[35]: <jpype._jclass.sun.misc.Launcher$AppClassLoader at 0x19e23b0>
In [36]: acl=_
In [37]: acl.ucp
Out[37]: <jpype._jclass.sun.misc.URLClassPath at 0x19e2c90>
In [48]: [u.toString() for u in acl.ucp.getURLs()]
Out[48]: [u'file:/C:/Documents%20and%20Settings/User/']
Now, URLClassPath has a public addURL method. Let's try it out and see what happens:
#normally, this is done with Launcher.getFileURL but we can't call it directly
#public static URLClassPath.pathToURLs also does the same, but it returns an array
In [72]: jpype.sun.net.www.ParseUtil.fileToEncodedURL(
jpype.java.io.File(r"c:\Ivan\downloads\dom4j-2.0.0-RC1.jar")
.getCanonicalFile())
Out[72]: <jpype._jclass.java.net.URL at 0x1a04b50>
In [73]: _.toString()
Out[73]: u'file:/C:/Ivan/downloads/dom4j-2.0.0-RC1.jar'
In [74]: acl.ucp.addURL(_72)
In [75]: [u.toString() for u in acl.ucp.getURLs()]
Out[75]:
[u'file:/C:/Documents%20and%20Settings/User/',
u'file:/C:/Ivan/downloads/dom4j-2.0.0-RC1.jar']
Now, let's try to load some class from the .jar:
In [78]: jpype.org=jpype._jpackage.JPackage("org")
In [79]: jpype.org.dom4j.Entity
Out[79]: jpype._jclass.org.dom4j.Entity
Success!
This will probably fail from a sandbox or such where there are custom class loaders or security settings in the way (AppClassLoader.loadClass does security checks before calling super).
Further code inspection shows that addURL also disables the URLClassPath's lookup cache (implemented in a few native methods), and this is irreversible. Initially, the lookupCacheEnabled flag is set to the value of the sun.cds.enableSharedLookupCache system property.
The interface provides no way to edit the entries. URLs are added to URLClassPath's private ArrayList path and Stack urls. urls is accessible, but it turns out, it only holds entries temporarily, before it's attempted to load from it, at which point the information moves to HashMap lmap and ArrayList loaders. getURLs() returns a copy of path. So, it's theoretically possible to edit it by hacking the accessible fields, but it's nowhere near reliable and won't affect getURLs result.
I can only comment from what I remember of my own experience of hacking a non-sun JVM ten years ago, but it did scan the entire classpath on startup, as an efficiency measure. The names of all classes found were added to an internal hashtable along with their locations (directory or zip/jar file). However, that was ten years ago and I can't help but wonder whether this would still be a reasonable thing to do in most settings considering how disk and memory architectures have evolved.
I believe that the classpath is taken to be static and the result of changing the files is undefined.
If you really want to be able to add and remove classes at runtime, consider doing so in your own classloader. This is what web containers do.
So shouldn't it be possible to add a
JAR or *.class file to the classpath
after the JVM started ...
You add jars and directories to the classpath, not classes. The classes are in either the directory, or the jar.
And if not, does that mean that the
classpath is searched on JVM startup
and all fully qualified names of the
found classes are cached in an
internal "list"?
That could be easily tested: Set the classpath, start your program, move a new class into the CP, call 'Class.forName ("NewClass") from your program. Does it find the new class?
I think you could read documentation of TomCat server. This server implements java classpapth by its own. So, when this server is started, you can deploy new webApp just drag and drop jar in appropriatefolder in hot, without restart server, and it will upload your app.