> ### 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...
Xueming Shen has updated the pull request incrementally with one additional commit since the last revision: test case update ------------- Changes: - all: https://git.openjdk.org/jdk/pull/27569/files - new: https://git.openjdk.org/jdk/pull/27569/files/23ada406..43b61040 Webrevs: - full: https://webrevs.openjdk.org/?repo=jdk&pr=27569&range=03 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27569&range=02-03 Stats: 127 lines in 1 file changed: 41 ins; 23 del; 63 mod Patch: https://git.openjdk.org/jdk/pull/27569.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/27569/head:pull/27569 PR: https://git.openjdk.org/jdk/pull/27569
