Classes used in method body are loaded lazily or eagerly depending on method return type

[David Holmes]

On 24/05/2023 12:50 am, Raffaello Giulietti wrote:
> I think the problem here is that you are deleting a class in a nest.
> 
> During the verification of BaseClassReturner.getObject(), access control 
> (JVMS, 5.4.4, second half) determines that the nest is broken, as 
> ChildClass is not present anymore.

Not sure access control gets involved at this stage of the verification 
process. But in any case turning on logging does not show anything 
related to nestmates happening between BaseClass and ChildClass. It 
seems to just be the resolution of the return type during verification 
of the method, that causes the loading of ChildClass and the subsequent 
CNFE if it has been removed.

> If you move ChildClass out of TestLoading so that it becomes a top-level 
> class extending TestLoading.BaseClass, and if you adapt the line that 
> initializes the local var classFileB to refer to the new location, the 
> code will not throw, despite ChildClass being deleted.

My simplified test shows it still throws when verifying BaseClassReturner.


Cheers,
David

> 
> Greetings
> Raffaello
> 
> 
> 
> On 2023-05-23 13:20, 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?
>>
>> 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();
>>      }
>>    }
>> }