javaClasses.hpp contains the C++ wrappers for a few dozens of Java classes. As more new features are developed using a combination of Java and C++, the list of classes is expected to grow.

(For example, recent addition of Loom classes not only increased the size of this header, but also caused more dependent header files to be included. See JDK-8288623.)

To improve modularity of the source code and reduce C++ build time, we should split the monolithic javaClasses.hpp into a new source directory under src/hotspot/share, where the wrapper classes are grouped by functional areas:

javaClasses/continuationClasses.hpp
    jdk_internal_vm_Continuation
    jdk_internal_vm_ContinuationScope
    jdk_internal_vm_StackChunk

javaClasses/coreClasses.hpp
    java_lang_AssertionStatusDirectives
    java_lang_Boolean
    java_lang_boxing_object
    java_lang_Byte
    java_lang_Byte_ByteCache
    java_lang_Character
    java_lang_Character_CharacterCache
    java_lang_Class
    java_lang_ClassLoader
    java_lang_Integer
    java_lang_Integer_IntegerCache
    java_lang_InternalError
    java_lang_Long
    java_lang_Long_LongCache
    java_lang_Object
    java_lang_Short
    java_lang_Short_ShortCache
    java_lang_String
    java_lang_System
    java_lang_Throwable
    java_security_AccessControlContext
    java_util_concurrent_locks_AbstractOwnableSynchronizer
    jdk_internal_misc_UnsafeConstants

javaClasses/foreignClasses.hpp
    jdk_internal_foreign_abi_ABIDescriptor
    jdk_internal_foreign_abi_CallConv
    jdk_internal_foreign_abi_NativeEntryPoint
    jdk_internal_foreign_abi_VMStorage

javaClasses/invokeClasses.hpp
    java_lang_invoke_CallSite
    java_lang_invoke_ConstantCallSite
    java_lang_invoke_DirectMethodHandle
    java_lang_invoke_LambdaForm
    java_lang_invoke_MemberName
    java_lang_invoke_MethodHandle
    java_lang_invoke_MethodHandleNatives_CallSiteContext
    java_lang_invoke_MethodType
    java_lang_invoke_ResolvedMethodName

javaClasses/referenceClasses.hpp
    java_lang_ref_Reference
    java_lang_ref_SoftReference

javaClasses/reflectionClasses.hpp
    java_lang_reflect_AccessibleObject
    java_lang_reflect_Constructor
    java_lang_reflect_Field
    java_lang_reflect_Method
    java_lang_reflect_RecordComponent

javaClasses/stackClasses.hpp
    java_lang_LiveStackFrameInfo
    java_lang_StackFrameInfo
    java_lang_StackTraceElement

javaClasses/threadClasses.hpp
    java_lang_Thread
    java_lang_Thread_Constants
    java_lang_Thread_FieldHolder
    java_lang_ThreadGroup
    java_lang_VirtualThread

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Alternatives:

We could use a single header file per wrapper class, like javaClasses/java_lang_String.hpp, but having 50+ little header files may be excessive.

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Risks:

- Source code change history may be lost.
- Backports may be more difficult

However, most of these classes just perform mechanical marshalling of parameters between C++ and Java, so the above risks may be minimal.