On 24/05/2023 5:04 am, Сергей Цыпанов wrote:
Hi,
Verification can require classes to be loaded to perform the
verification - see JVMS 4.10 for all the details.
sorry, I still don't get it completely. Here's the byte code for
ObjectReturner.getObject():
// access flags 0x1
public getObject()Ljava/lang/Object;
L0
LINENUMBER 43 L0
NEW org/example/TestLoading$ChildClass
DUP
INVOKESPECIAL org/example/TestLoading$ChildClass.<init> ()V
ARETURN
L1
LOCALVARIABLE this Lorg/example/TestLoading$ObjectReturner; L0 L1 0
MAXSTACK = 2
MAXLOCALS = 1
and this one is for BaseClassReturner.getObject():
// access flags 0x1
public getObject()Lorg/example/TestLoading$BaseClass;
L0
LINENUMBER 49 L0
NEW org/example/TestLoading$ChildClass
DUP
INVOKESPECIAL org/example/TestLoading$ChildClass.<init> ()V
ARETURN
L1
LOCALVARIABLE this Lorg/example/TestLoading$BaseClassReturner; L0 L1 0
MAXSTACK = 2
MAXLOCALS = 1
Apart from type of 'this' the only difference is return type, so I've
referenced JVMS 4.10 in the part where return type is described.
There we have clause for areturn:
"An areturn instruction is type safe iff the enclosing method has a declared return
type, ReturnType, that is a reference type, and one can validly pop a type matching
ReturnType off the incoming operand stack."
and for return type:
"If the method returns a reference type, only an areturn instruction may be used,
and the type of the returned value must be assignment compatible with the return
descriptor of the method (§4.3.3)"
I guess the second one is a clue for to check whether returned value can be
assigned to return type the VM should look into inheritance tree. Please
correct me if idea is wrong.
For the Object case the type-checking is always obviously trivially
correct so it doesn't need to load the actual return type. So the seems
a reasonable conclusion.
If you take your test code and only have the BaseClassReturner and only
load it then you see in the logging:
loading: TestLoading$BaseClassReturner...
[0.067s][info][class,init ] Start class verification for:
TestLoading$BaseClassReturner
[0.067s][info][verification ] Verifying class
TestLoading$BaseClassReturner with new format
[0.067s][info][verification ] Verifying method
TestLoading$BaseClassReturner.<init>()V
[0.067s][info][verification ] Verifying method
TestLoading$BaseClassReturner.getObject()LTestLoading$BaseClass;
[0.068s][info][class,load ] TestLoading$BaseClass source:
file:/scratch/users/daholme/tests/
[0.068s][info][class,load ] TestLoading$ChildClass source:
file:/scratch/users/daholme/tests/
[0.068s][info][class,init ] End class verification for:
TestLoading$BaseClassReturner
[0.068s][info][verification ] End class verification for:
TestLoading$BaseClassReturner
So you can see the ChildClass being loaded.
David
-----
Regards,
Sergey
Hi,
On 23/05/2023 9:20 pm, Сергей Цыпанов wrote:
Hello,
originally this question was asked here:
https://stackoverflow.com/q/76260269/12473843,
the code sample for reproduction will be put below or can be found via the link
The issue is about eager/lazy loading of a class depending on method return
type.
If one runs the code below with Java 11-19 it will fail with
NoClassDefFoundError (this is expected as delete class file for ChildClass):
java.lang.NoClassDefFoundError: org/example/TestLoading$ChildClass
at java.base/java.lang.Class.forName0(Native Method)
at java.base/java.lang.Class.forName(Class.java:390)
at java.base/java.lang.Class.forName(Class.java:381)
at org.example.TestLoading.loadMyClass(TestLoading.java:29)
at org.example.TestLoading.main(TestLoading.java:23)
Caused by: java.lang.ClassNotFoundException: org.example.TestLoading$ChildClass
at
java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641)
at
java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:521)
... 5 more
As of Java 20 chapter 12.4.1 of JLS states:
---------------------------------------------------------
A class or interface T will be initialized immediately before the first
occurrence of any one of the following:
- T is a class and an instance of T is created.
- a static method declared by T is invoked.
- a static field declared by T is assigned.
- a static field declared by T is used and the field is not a constant variable
(§4.12.4).
When a class is initialized, its superclasses are initialized (if they have not
been previously initialized), as well as any superinterfaces (§8.1.5) that
declare any default methods (§9.4.3) (if they have not been previously
initialized).
Initialization of an interface does not, of itself, cause initialization of any
of its superinterfaces.
A reference to a static field (§8.3.1.1) causes initialization of only the
class or interface that actually declares it, even though it might be referred
to through the name of a subclass, a subinterface, or a class that implements
an interface.
Invocation of certain reflective methods in class Class and in package
java.lang.reflect also causes class or interface initialization.
A class or interface will not be initialized under any other circumstance.
---------------------------------------------------------
With the code snippet we see that calling
Class.forName(ObjectReturner.class.getName()) succeeds and
Class.forName(BaseClassReturner.class.getName()) fails even though both declare
returning an instance of ChildClass.
This failure is unexpected as in the code below we don't fulfill any
requirement for class loading as of JLS 12.4.1, but the JVM still tries to load
the class.
I suspect it might be related to class file validation and/or security, because
when we run the code with -Xlog:class+init there's a reference to LinkageError
in loading log:
loading: org.example.TestLoading$BaseClassReturner...
[0.277s][info][class,init] Start class verification for:
org.example.TestLoading$BaseClassReturner
[0.277s][info][class,init] 771 Initializing
'java/lang/ReflectiveOperationException'(no method) (0x0000000800004028)
[0.277s][info][class,init] 772 Initializing
'java/lang/ClassNotFoundException'(no method) (0x0000000800004288)
[0.277s][info][class,init] 773 Initializing 'java/lang/LinkageError'(no method)
(0x00000008000044f8) <----
[0.277s][info][class,init] 774 Initializing 'java/lang/NoClassDefFoundError'(no
method) (0x0000000800004758)
[0.277s][info][class,init] Verification for
org.example.TestLoading$BaseClassReturner has exception pending
'java.lang.NoClassDefFoundError org/example/TestLoading$ChildClass'
[0.277s][info][class,init] End class verification for:
org.example.TestLoading$BaseClassReturner
So I've got three questions about this:
- Does class loading depend on method's return type?
- Which part of JLS/JVM spec describes eager class loading in this case?
- Could one point out the particular piece of the VM code responsible for class
loading in this case?
Verification can require classes to be loaded to perform the
verification - see JVMS 4.10 for all the details.
Note you seem to be confusing class loading with class initialization
above. The rules for initialization are very precise; the rules for
loading are far more lax.
Cheers,
David
-----
Regards,
Sergey Tsypanov
Code snippet for reproduction:
public class TestLoading {
public static void main(String[] args) throws Exception {
Class.forName(BaseClass.class.getName());
URL classFileB = TestLoading.class.getResource(TestLoading.class.getSimpleName() +
"$ChildClass.class");
if (classFileB != null) {
if (!"file".equals(classFileB.getProtocol())) {
throw new UnsupportedOperationException();
}
Path path = new File(classFileB.getPath()).toPath();
System.out.println("deleting: " + path);
Files.delete(path);
}
loadMyClass(ObjectReturner.class.getName());
loadMyClass(BaseClassReturner.class.getName());
}
private static void loadMyClass(String name) {
System.out.println("loading: " + name + "...");
try {
Class.forName(name);
} catch (Throwable e) {
e.printStackTrace(System.out);
}
}
public static class BaseClass {
}
public static class ChildClass extends BaseClass {
}
public static class ObjectReturner {
public Object getObject() {
return new ChildClass();
}
}
public static class BaseClassReturner {
public BaseClass getObject() {
return new ChildClass();
}
}
}
