On Wed, 18 Nov 2020 00:50:22 GMT, Mandy Chung <mch...@openjdk.org> wrote:
>> Provide the `Lookup::defineHiddenClassWithClassData` API that allows live >> objects >> be shared between a hidden class and other classes. A hidden class can load >> these live objects as dynamically-computed constants via this API. >> >> Specdiff >> http://cr.openjdk.java.net/~mchung/jdk16/webrevs/8230501/specdiff/overview-summary.html >> >> With this class data support and hidden classes, >> `sun.misc.Unsafe::defineAnonymousClass` >> will be deprecated for removal. Existing libraries should replace their >> calls to `sun.misc.Unsafe::defineAnonymousClass` with >> `Lookup::defineHiddenClass` >> or `Lookup::defineHiddenClassWithClassData`. >> >> This patch also updates the implementation of lambda meta factory and >> `MemoryAccessVarHandleGenerator` to use class data. No performance >> difference >> observed in the jdk.incubator.foreign microbenchmarks. A side note: >> `MemoryAccessVarHandleGenerator` is removed in the upcoming integration of >> JDK-8254162 but it helps validating the class data support. >> >> Background >> ---------- >> >> This is an enhancement following up JEP 371: Hidden Classes w.r.t. >> "Constant-pool patching" in the "Risks and Assumption" section. >> >> A VM-anonymous class can be defined with its constant-pool entries already >> resolved to concrete values. This allows critical constants to be shared >> between a VM-anonymous class and the language runtime that defines it, and >> between multiple VM-anonymous classes. For example, a language runtime will >> often have `MethodHandle` objects in its address space that would be useful >> to newly-defined VM-anonymous classes. Instead of the runtime serializing >> the objects to constant-pool entries in VM-anonymous classes and then >> generating bytecode in those classes to laboriously `ldc` the entries, >> the runtime can simply supply `Unsafe::defineAnonymousClass` with references >> to its live objects. The relevant constant-pool entries in the newly-defined >> VM-anonymous class are pre-linked to those objects, improving performance >> and reducing footprint. In addition, this allows VM-anonymous classes to >> refer to each other: Constant-pool entries in a class file are based on >> names. >> They thus cannot refer to nameless VM-anonymous classes. A language runtime >> can, >> however, easily track the live Class objects for its VM-anonymous classes and >> supply them to `Unsafe::defineAnonymousClass`, thus pre-linking the new >> class's >> constant pool entries to other VM-anonymous classes. >> >> This extends the hidden classes to allow live objects to be injected >> in a hidden class and loaded them via condy. >> >> Details >> ------- >> >> A new `Lookup::defineHiddenClassWithClassData` API takes additional >> `classData` argument compared to `Lookup::defineHiddenClass`. >> Class data can be method handles, lookup objects, arbitrary user objects >> or collections of all of the above. >> >> This method behaves as if calling `Lookup::defineHiddenClass` to define >> a hidden class with a private static unnamed field that is initialized >> with `classData` at the first instruction of the class initializer. >> >> `MethodHandles::classData(Lookup lookup, String name, Class<?> type)` and >> `MethodHandles::classDataAt(Lookup lookup, String name, Class<?> type, int >> index)` >> are the bootstrap methods to load the class data of the given lookup's >> lookup class. >> The hidden class will be initialized when `classData` method is called if >> the hidden class has not been initialized. >> >> For a class data containing more than one single element, libraries can >> create their convenience method to load a single live object via condy. >> >> Frameworks sometimes want to dynamically create a hidden class (HC) and add >> it >> it the lookup class nest and have HC to carry secrets hidden from that nest. >> In this case, frameworks should not to use private static finals (in the HCs >> they spin) to hold secrets because a nestmate of HC may obtain access to >> such a private static final and observe the framework's secret. It should >> use >> condy. In addition, we need to differentiate if a lookup object is created >> from >> the original lookup class or created from teleporting e.g. `Lookup::in` >> and `MethodHandles::privateLookupIn`. >> >> This proposes to add a new `ORIGINAL` bit that is only set if the lookup >> object is created by `MethodHandles::lookup` or by bootstrap method >> invocation. >> The operations only apply to a Lookup object with original access are: >> - create method handles for caller-sensitve methods >> - obtain class data associated with the lookup class >> >> No change to `Lookup::hasFullPrivilegeAccess` and `Lookup::toString` which >> ignores the ORIGINAL bit. >> >> >> Compatibility Risks >> ------------------- >> >> `Lookup::lookupModes` includes a new `ORIGINAL` bit. Most lookup operations >> ignore this original bit except creating method handles for caller-sensitive >> methods >> that expects the lookup from the original lookup class. Existing code >> compares >> the return value of `lookupModes` to be a fixed value may be impacted. >> However >> existing client has no need to expect a fixed value of lookup modes. >> The incompatibility risk of this spec change is low. > > Mandy Chung has updated the pull request incrementally with one additional > commit since the last revision: > > Fix the name passed to condy calling classData IIUC `classData` can be used for an original lookup that is not produced by the result of `defineHiddenClassWithClassData`, but in such cases the class data will always be null. Since `defineHiddenClassWithClassData` rejects null values for class data, we could detect such usage and throw in the bootstrap methods. That would require a special constant assignment for hidden classes with no class data. Probably not worth it. Recommend an API note. src/java.base/share/classes/java/lang/invoke/MethodHandles.java line 342: > 340: } catch (ClassCastException e) { > 341: throw e; > 342: } catch (Throwable e) { The following might be more appropriate so, in general, errors and runtime exceptions are not explicitly wrapped: try { return BootstrapMethodInvoker.widenAndCast(classdata, type); } catch (RuntimeException | Error e) { throw e; } catch (Throwable e) { throw new InternalError("Unexpected exception", e); } same applies to `classDataAt` and `ConstantBootstraps.explicitCast`. Refinement of the runtime exceptions is also possible, but i think the key thing here is to let errors pass through and any possibly expected runtime exceptions will get wrapped in `BootstrapMethodError`. test/jdk/java/lang/invoke/MethodHandles/classData/ClassDataTest.java line 77: > 75: */ > 76: @Test > 77: public void noClassData() throws Throwable { `throws Throwable` needed on this and other method declarations? ------------- Marked as reviewed by psandoz (Reviewer). PR: https://git.openjdk.java.net/jdk/pull/1171
