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8373202: [lworld] ObjectReference.equals should follow == semantics for value objects #1834

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8373202: [lworld] ObjectReference.equals should follow == semantics for value objects #1834

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1d883c2
ctor_debug
alexmenkov Dec 18, 2025
207aca4
Revert "ctor_debug"
alexmenkov Dec 18, 2025
88ac48b
fix
alexmenkov Dec 18, 2025
ff1803d
tabs
alexmenkov Dec 18, 2025
32ea988
Merge branch 'openjdk:lworld' into lworld
alexmenkov Dec 18, 2025
a5edf06
Merge branch 'openjdk:lworld' into lworld
alexmenkov Jan 5, 2026
c46ea6a
Merge branch 'openjdk:lworld' into lworld
alexmenkov Jan 8, 2026
576fc6a
Merge branch 'openjdk:lworld' into lworld
alexmenkov Jan 9, 2026
549e6a7
fix
alexmenkov Dec 18, 2025
96b3321
tabs
alexmenkov Dec 18, 2025
d58eae4
IsSameObject only for value objects
alexmenkov Jan 8, 2026
e0b4df7
Merge branch 'ctor_debug' of github.com:alexmenkov/valhalla into ctor…
alexmenkov Jan 10, 2026
f82f5be
ObjectReferenceImp.hashCode
alexmenkov Jan 12, 2026
e74a4cf
ClassTypeImpl.isValueClass, VirtualMachineImpl.canUseIsSameObject
alexmenkov Jan 15, 2026
34c9c2b
test update
alexmenkov Jan 16, 2026
55ba146
space
alexmenkov Jan 16, 2026
583f9e3
feedback
alexmenkov Jan 22, 2026
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37 changes: 37 additions & 0 deletions src/java.se/share/data/jdwp/jdwp.spec
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Original file line number Diff line number Diff line change
Expand Up @@ -1801,6 +1801,43 @@ JDWP "Java(tm) Debug Wire Protocol"
(Error VM_DEAD)
)
)
(Command IsSameObject=11
"<b>IsSameObject is a preview API of the Java platform.</b> "
"<em>Preview features may be removed in a future release, or upgraded to "
"permanent features of the Java platform.</em>"
"<p>"
"Determines whether two objects refer to the same Java object."
"<p>Since JDWP version 27."
(Out
(object object1 "The object ID")
(object object2 "The object ID")
)
(Reply
(boolean isSameObject "true if the objects refer to the same Java object; false otherwise")
)
(ErrorSet
(Error INVALID_OBJECT)
(Error VM_DEAD)
)
)
(Command ObjectHashCode=12
"<b>ObjectHashCode is a preview API of the Java platform.</b> "
"<em>Preview features may be removed in a future release, or upgraded to "
"permanent features of the Java platform.</em>"
"<p>"
"Returns hash code for an object."
"<p>Since JDWP version 27."
(Out
(object object "The object ID")
)
(Reply
(int hashCode "hash code value for the object")
)
(ErrorSet
(Error INVALID_OBJECT)
(Error VM_DEAD)
)
)
)

(CommandSet StringReference=10
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10 changes: 10 additions & 0 deletions src/jdk.jdi/share/classes/com/sun/tools/jdi/ClassTypeImpl.java
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Original file line number Diff line number Diff line change
Expand Up @@ -131,6 +131,16 @@ public boolean isEnum() {
return false;
}

public boolean isValueClass() {
if (!vm.supportsValueClasses()) {
return false;
}
if (modifiers == -1) {
getModifiers();
}
return (modifiers & VMModifiers.IDENTITY) == 0;
}

public void setValue(Field field, Value value)
throws InvalidTypeException, ClassNotLoadedException {

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33 changes: 29 additions & 4 deletions src/jdk.jdi/share/classes/com/sun/tools/jdi/ObjectReferenceImpl.java
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Original file line number Diff line number Diff line change
Expand Up @@ -145,17 +145,42 @@ public boolean vmNotSuspended(VMAction action) {
}
}

boolean isValueObject() {
if (referenceType() instanceof ClassTypeImpl classType) {
return classType.isValueClass();
}
return false;
}

public boolean equals(Object obj) {
if (obj instanceof ObjectReferenceImpl other) {
return (ref() == other.ref()) &&
super.equals(obj);
} else {
return false;
if (!super.equals(obj)) { // checks if the references belong to the same VM
return false;
}
if (ref() == other.ref()) {
return true;
}
// We can get equal value objects with different IDs.
if (isValueObject()) {
try {
return JDWP.ObjectReference.IsSameObject.process(vm, this, other).isSameObject;
} catch (JDWPException exc) {
throw exc.toJDIException();
}
}
}
return false;
}

@Override
public int hashCode() {
if (isValueObject()) {
try {
return JDWP.ObjectReference.ObjectHashCode.process(vm, this).hashCode;
} catch (JDWPException exc) {
throw exc.toJDIException();
}
}
return Long.hashCode(ref());
}

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1 change: 1 addition & 0 deletions src/jdk.jdi/share/classes/com/sun/tools/jdi/VMModifiers.java
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Expand Up @@ -32,6 +32,7 @@ public interface VMModifiers {
int STATIC = 0x00000008; /* instance variable is static */
int FINAL = 0x00000010; /* no further subclassing, overriding */
int SYNCHRONIZED = 0x00000020; /* wrap method call in monitor lock */
int IDENTITY = 0x0020; /* identity (not value) class */
int VOLATILE = 0x00000040; /* can cache in registers */
int BRIDGE = 0x00000040; /* Bridge method generated by compiler */
int TRANSIENT = 0x00000080; /* not persistent */
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4 changes: 4 additions & 0 deletions src/jdk.jdi/share/classes/com/sun/tools/jdi/VirtualMachineImpl.java
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Expand Up @@ -835,6 +835,10 @@ boolean mayCreateVirtualThreads() {
return versionInfo().jdwpMajor >= 19;
}

public boolean supportsValueClasses() {
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@plummercj plummercj Jan 23, 2026

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What happens if not running in preview mode? Value classes are not supported in that case, but as implemented this API will say they are supported, leading to isValueClass() looking at modifiers in a way that is not compatible when value classes are not supported.

JDI didn't really run into this type of problem with virtual thread support. It did modify a couple of new public APIs with:

@PreviewFeature(feature = PreviewFeature.Feature.VIRTUAL_THREADS)

However, I don't see how that helps us here. It seems we need to query the debug agent to find out if value classes are supported. There's no good way to do that. Is there any way for JDI to somewhat detect that value class support is supported (preview mode is enabled).

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@sspitsyn sspitsyn Jan 23, 2026

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What happens if not running in preview mode? Value classes are not supported in that case, but as implemented this API will say they are supported, leading to isValueClass() looking at modifiers in a way that is not compatible when value classes are not supported.

It should work correctly. The IDENTITY modifier bit is not used for other purposes in JDK 27 (Alex, please, fix me if it is wrong). So, the check for version >= 27 is needed. If preview is not enabled in target VM, so it does not support value classes then the IDENTITY modifier bit will be always set which results in isValueClass() to always return false.

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@plummercj plummercj Jan 23, 2026

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So in JDK 27, with or without enable preview, the IDENTITY modifier bit is accurate and can be looked at, but in previous JDK versions it might be used for something else?

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@alexmenkov alexmenkov Jan 23, 2026

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So in JDK 27, with or without enable preview, the IDENTITY modifier bit is accurate and can be looked at, but in previous JDK versions it might be used for something else?

Correct

return versionInfo().jdwpMajor >= 27;
}

public void setDebugTraceMode(int traceFlags) {
validateVM();
this.traceFlags = traceFlags;
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61 changes: 60 additions & 1 deletion src/jdk.jdwp.agent/share/native/libjdwp/ObjectReferenceImpl.c
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Expand Up @@ -357,6 +357,63 @@ referringObjects(PacketInputStream *in, PacketOutputStream *out)
return JNI_TRUE;
}

static jboolean
isSameObjectImpl(PacketInputStream *in, PacketOutputStream *out)
{
jlong id1;
jlong id2;
jobject ref1;
jobject ref2;
JNIEnv *env;

env = getEnv();
id1 = inStream_readObjectID(in);
id2 = inStream_readObjectID(in);
if (inStream_error(in)) {
return JNI_TRUE;
}

if (id1 == NULL_OBJECT_ID || id2 == NULL_OBJECT_ID) {
outStream_setError(out, JDWP_ERROR(INVALID_OBJECT));
return JNI_TRUE;
}

ref1 = commonRef_idToRef(env, id1);
ref2 = commonRef_idToRef(env, id2);
(void)outStream_writeBoolean(out, isSameObject(env, ref1, ref2));

commonRef_idToRef_delete(env, ref1);
commonRef_idToRef_delete(env, ref2);

return JNI_TRUE;
}

static jboolean
objectHashCodeImpl(PacketInputStream *in, PacketOutputStream *out)
{
jlong id;
jobject ref;
JNIEnv *env;

env = getEnv();
id = inStream_readObjectID(in);
if (inStream_error(in)) {
return JNI_TRUE;
}

if (id == NULL_OBJECT_ID) {
outStream_setError(out, JDWP_ERROR(INVALID_OBJECT));
return JNI_TRUE;
}

ref = commonRef_idToRef(env, id);
(void)outStream_writeInt(out, objectHashCode(ref));

commonRef_idToRef_delete(env, ref);

return JNI_TRUE;
}

Command ObjectReference_Commands[] = {
{referenceType, "ReferenceType"},
{getValues, "GetValues"},
Expand All @@ -367,7 +424,9 @@ Command ObjectReference_Commands[] = {
{disableCollection, "DisableCollection"},
{enableCollection, "EnableCollection"},
{isCollected, "IsCollected"},
{referringObjects, "ReferringObjects"}
{referringObjects, "ReferringObjects"},
{isSameObjectImpl, "IsSameObject"},
{objectHashCodeImpl, "ObjectHashCode"}
};

DEBUG_DISPATCH_DEFINE_CMDSET(ObjectReference)
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