On Tue, 30 Sep 2025 07:54:20 GMT, Xueming Shen <[email protected]> wrote:
> ### Background > > - ClassLoader.defineClass can receive class data in the form of arrays or > ByteBuffers. > - For array-backed data (defineClass1), a defensive copy is made before > passing it to JVM_DefineClassWithSource(). > - For Direct-ByteBuffer variants (defineClass2), no defensive copy is made, > which creates a risk that the underlying bytes could be modified while the > JVM is processing them. > - Although a caller could always modify a buffer before a defensive copy is > made — a race condition that cannot be completely prevented — the **_main > concern_** is ensuring that the JVM never processes class bytes that are > being concurrently modified. > > ### Problem > > - Concurrent modification risk during processing: while we cannot prevent > pre-copy modifications, we **_must prevent the JVM from using class bytes > that are being modified concurrently._** > - Performance concerns: defensive copies have a cost, especially for direct > byte buffers. Making copies unnecessarily for trusted class loaders (like the > built-in class loader) would hurt performance. > > ### Solution > > - Make a defensive copy of the direct byte-buffer only when the class loader > is **NOT** a built-in/trusted class loader. > - For the built-in class loader, skip the copy because the JVM can guarantee > that the buffer contents remain intact. > > This approach ensures the integrity of class bytes processes for untrusted > or custom class loaders while minimizing performance impact for trusted or > built-in loaders. > > ### Benchmark > > A JMH benchmark has been added to measure the potential cost of the defensive > copy. The results indicate that the performance impact is minimal and largely > insignificant. > > **Before:** > > > Benchmark Mode Cnt Score > Error Units > ClassLoaderDefineClass.testDefineClassByteBufferDirect avgt 15 8387.247 ± > 1405.681 ns/op > ClassLoaderDefineClass.testDefineClassByteBufferHeap avgt 15 8971.739 ± > 1020.288 ns/op > Finished running test > 'micro:org.openjdk.bench.java.lang.ClassLoaderDefineClass' > Test report is stored in > /Users/xuemingshen/jdk26/build/macosx-aarch64/test-results/micro_org_openjdk_bench_java_lang_ClassLoaderDefineClass > > > **After:** > > > Benchmark Mode Cnt Score > Error Units > ClassLoaderDefineClass.testDefineClassByteBufferDirect avgt 15 8521.881 ± > 2002.011 ns/op > ClassLoaderDefineClass.testDefineClassByteBufferHeap avgt 15 9002.842 ± > 1099.635 ns/op > Finished running test 'mi... This pull request has now been integrated. Changeset: 4ca4485e Author: Xueming Shen <[email protected]> URL: https://git.openjdk.org/jdk/commit/4ca4485e9af10a49ca95710c4e26aa3895835d47 Stats: 432 lines in 3 files changed: 427 ins; 0 del; 5 mod 8365588: defineClass that accepts a ByteBuffer does not work as expected Reviewed-by: alanb ------------- PR: https://git.openjdk.org/jdk/pull/27569
