CoreLibs invokedynamic字节码指北

VM层

templateInterpreter的invokedynamic如下：

void TemplateTable::invokedynamic(int byte_no) {
  transition(vtos, vtos);
  assert(byte_no == f1_byte, "use this argument");

  const Register rbx_method   = rbx;
  const Register rax_callsite = rax;

  prepare_invoke(byte_no, rbx_method, rax_callsite);

  // rax: CallSite object (from cpool->resolved_references[f1])
  // rbx: MH.linkToCallSite method (from f2)

  // Note:  rax_callsite is already pushed by prepare_invoke

  // %%% should make a type profile for any invokedynamic that takes a ref argument
  // profile this call
  __ profile_call(rbcp);
  __ profile_arguments_type(rdx, rbx_method, rbcp, false);

  __ verify_oop(rax_callsite);

  __ jump_from_interpreted(rbx_method, rdx);
}


void TemplateTable::prepare_invoke(int byte_no,
                                   Register method,  // linked method (or i-klass)
                                   Register index,   // itable index, MethodType, etc.
                                   Register recv,    // if caller wants to see it
                                   Register flags    // if caller wants to test it
                                   ) {
  // determine flags
  const Bytecodes::Code code = bytecode();
  const bool is_invokeinterface  = code == Bytecodes::_invokeinterface;
  const bool is_invokedynamic    = code == Bytecodes::_invokedynamic;
  const bool is_invokehandle     = code == Bytecodes::_invokehandle;
  const bool is_invokevirtual    = code == Bytecodes::_invokevirtual;
  const bool is_invokespecial    = code == Bytecodes::_invokespecial;
  const bool load_receiver       = (recv  != noreg);
  const bool save_flags          = (flags != noreg);
  assert(load_receiver == (code != Bytecodes::_invokestatic && code != Bytecodes::_invokedynamic), "");
  assert(save_flags    == (is_invokeinterface || is_invokevirtual), "need flags for vfinal");
  assert(flags == noreg || flags == rdx, "");
  assert(recv  == noreg || recv  == rcx, "");

  // setup registers & access constant pool cache
  if (recv  == noreg)  recv  = rcx;
  if (flags == noreg)  flags = rdx;
  assert_different_registers(method, index, recv, flags);

  // save 'interpreter return address'
  __ save_bcp();

  load_invoke_cp_cache_entry(byte_no, method, index, flags, is_invokevirtual, false, is_invokedynamic);

  // maybe push appendix to arguments (just before return address)
  if (is_invokedynamic || is_invokehandle) {
    Label L_no_push;
    __ testl(flags, (1 << ConstantPoolCacheEntry::has_appendix_shift));
    __ jcc(Assembler::zero, L_no_push);
    // Push the appendix as a trailing parameter.
    // This must be done before we get the receiver,
    // since the parameter_size includes it.
    __ push(rbx);
    __ mov(rbx, index);
    assert(ConstantPoolCacheEntry::_indy_resolved_references_appendix_offset == 0, "appendix expected at index+0");
    __ load_resolved_reference_at_index(index, rbx);
    __ pop(rbx);
    __ push(index);  // push appendix (MethodType, CallSite, etc.)
    __ bind(L_no_push);
  }

  // load receiver if needed (after appendix is pushed so parameter size is correct)
  // Note: no return address pushed yet
  if (load_receiver) {
    __ movl(recv, flags);
    __ andl(recv, ConstantPoolCacheEntry::parameter_size_mask);
    const int no_return_pc_pushed_yet = -1;  // argument slot correction before we push return address
    const int receiver_is_at_end      = -1;  // back off one slot to get receiver
    Address recv_addr = __ argument_address(recv, no_return_pc_pushed_yet + receiver_is_at_end);
    __ movptr(recv, recv_addr);
    __ verify_oop(recv);
  }

  if (save_flags) {
    __ movl(rbcp, flags);
  }

  // push return address
  __ push(flags);

  // Restore flags value from the constant pool cache, and restore rsi
  // for later null checks.  r13 is the bytecode pointer
  if (save_flags) {
    __ movl(flags, rbcp);
    __ restore_bcp();
  }

  // compute return type
  __ shrl(flags, ConstantPoolCacheEntry::tos_state_shift);
  // Make sure we don't need to mask flags after the above shift
  ConstantPoolCacheEntry::verify_tos_state_shift();
  // load return address
  {
    const address table_addr = (address) Interpreter::invoke_return_entry_table_for(code);
    ExternalAddress table(table_addr);
    LP64_ONLY(__ lea(rscratch1, table));
    LP64_ONLY(__ movptr(flags, Address(rscratch1, flags, Address::times_ptr)));
    NOT_LP64(__ movptr(flags, ArrayAddress(table, Address(noreg, flags, Address::times_ptr))));
  }
}

void TemplateTable::load_invoke_cp_cache_entry(int byte_no,
                                               Register method,
                                               Register itable_index,
                                               Register flags,
                                               bool is_invokevirtual,
                                               bool is_invokevfinal, /*unused*/
                                               bool is_invokedynamic) {
  // setup registers
  const Register cache = rcx;
  const Register index = rdx;
  assert_different_registers(method, flags);
  assert_different_registers(method, cache, index);
  assert_different_registers(itable_index, flags);
  assert_different_registers(itable_index, cache, index);
  // determine constant pool cache field offsets
  assert(is_invokevirtual == (byte_no == f2_byte), "is_invokevirtual flag redundant");
  const int method_offset = in_bytes(
    ConstantPoolCache::base_offset() +
      ((byte_no == f2_byte)
       ? ConstantPoolCacheEntry::f2_offset()
       : ConstantPoolCacheEntry::f1_offset()));
  const int flags_offset = in_bytes(ConstantPoolCache::base_offset() +
                                    ConstantPoolCacheEntry::flags_offset());
  // access constant pool cache fields
  const int index_offset = in_bytes(ConstantPoolCache::base_offset() +
                                    ConstantPoolCacheEntry::f2_offset());

  size_t index_size = (is_invokedynamic ? sizeof(u4) : sizeof(u2));
  resolve_cache_and_index(byte_no, cache, index, index_size);
    __ movptr(method, Address(cache, index, Address::times_ptr, method_offset));

  if (itable_index != noreg) {
    // pick up itable or appendix index from f2 also:
    __ movptr(itable_index, Address(cache, index, Address::times_ptr, index_offset));
  }
  __ movl(flags, Address(cache, index, Address::times_ptr, flags_offset));
}

void TemplateTable::resolve_cache_and_index(int byte_no,
                                            Register Rcache,
                                            Register index,
                                            size_t index_size) {
  const Register temp = rbx;
  assert_different_registers(Rcache, index, temp);

  Label resolved;

  Bytecodes::Code code = bytecode();
  switch (code) {
  case Bytecodes::_nofast_getfield: code = Bytecodes::_getfield; break;
  case Bytecodes::_nofast_putfield: code = Bytecodes::_putfield; break;
  default: break;
  }

  assert(byte_no == f1_byte || byte_no == f2_byte, "byte_no out of range");
  __ get_cache_and_index_and_bytecode_at_bcp(Rcache, index, temp, byte_no, 1, index_size);
  __ cmpl(temp, code);  // have we resolved this bytecode?
  __ jcc(Assembler::equal, resolved);

  // resolve first time through
  address entry = CAST_FROM_FN_PTR(address, InterpreterRuntime::resolve_from_cache);
  __ movl(temp, code);
  __ call_VM(noreg, entry, temp);
  // Update registers with resolved info
  __ get_cache_and_index_at_bcp(Rcache, index, 1, index_size);
  __ bind(resolved);
}

void TemplateTable::resolve_cache_and_index(int byte_no,
                                            Register Rcache,
                                            Register index,
                                            size_t index_size) {
  const Register temp = rbx;
  assert_different_registers(Rcache, index, temp);

  Label resolved;

  Bytecodes::Code code = bytecode();
  switch (code) {
  case Bytecodes::_nofast_getfield: code = Bytecodes::_getfield; break;
  case Bytecodes::_nofast_putfield: code = Bytecodes::_putfield; break;
  default: break;
  }

  assert(byte_no == f1_byte || byte_no == f2_byte, "byte_no out of range");
  __ get_cache_and_index_and_bytecode_at_bcp(Rcache, index, temp, byte_no, 1, index_size);
  __ cmpl(temp, code);  // have we resolved this bytecode?
  __ jcc(Assembler::equal, resolved);

  // resolve first time through
  address entry = CAST_FROM_FN_PTR(address, InterpreterRuntime::resolve_from_cache);
  __ movl(temp, code);
  __ call_VM(noreg, entry, temp);
  // Update registers with resolved info
  __ get_cache_and_index_at_bcp(Rcache, index, 1, index_size);
  __ bind(resolved);
}

TemplateTable::invokedynamic由两步组成，第一步prepare_invoke将要调用的方法放到rbx，然后jump_from_interpreted跳到这个rbx方法入口。所以invokedynamic的所有故事主要发生在prepare_invoke。

prepare_invoke一步步会调用到InterpreterRuntime::resolve_from_cache，注意，从prepare_invoke到resolve_from_cache都是模板解释器的JIT代码，到resolve_from_cache就是VM代码了。resolve_from_cache是几个invoke*方法共同的入口：

IRT_ENTRY(void, InterpreterRuntime::resolve_from_cache(JavaThread* thread, Bytecodes::Code bytecode)) {
  switch (bytecode) {
  case Bytecodes::_getstatic:
  case Bytecodes::_putstatic:
  case Bytecodes::_getfield:
  case Bytecodes::_putfield:
    resolve_get_put(thread, bytecode);
    break;
  case Bytecodes::_invokevirtual:
  case Bytecodes::_invokespecial:
  case Bytecodes::_invokestatic:
  case Bytecodes::_invokeinterface:
    resolve_invoke(thread, bytecode);
    break;
  case Bytecodes::_invokehandle:
    resolve_invokehandle(thread);
    break;
  case Bytecodes::_invokedynamic:
    resolve_invokedynamic(thread);
    break;
  default:
    fatal("unexpected bytecode: %s", Bytecodes::name(bytecode));
    break;
  }
}
IRT_END

这里主要关注InterpreterRuntime::resolve_invokedynamic。它会一步步走下去，最终走到LinkResolver::resolve_invokedynamic：

void LinkResolver::resolve_invokedynamic(CallInfo& result, const constantPoolHandle& pool, int index, TRAPS) {
  Symbol* method_name       = pool->name_ref_at(index);
  Symbol* method_signature  = pool->signature_ref_at(index);
  Klass* current_klass = pool->pool_holder();

  // Resolve the bootstrap specifier (BSM + optional arguments).
  Handle bootstrap_specifier;
  // Check if CallSite has been bound already:
  ConstantPoolCacheEntry* cpce = pool->invokedynamic_cp_cache_entry_at(index);
  int pool_index = cpce->constant_pool_index();

  if (cpce->is_f1_null()) {
    if (cpce->indy_resolution_failed()) {
      ConstantPool::throw_resolution_error(pool,
                                           ResolutionErrorTable::encode_cpcache_index(index),
                                           CHECK);
    }

    // The initial step in Call Site Specifier Resolution is to resolve the symbolic
    // reference to a method handle which will be the bootstrap method for a dynamic
    // call site.  If resolution for the java.lang.invoke.MethodHandle for the bootstrap
    // method fails, then a MethodHandleInError is stored at the corresponding bootstrap
    // method's CP index for the CONSTANT_MethodHandle_info.  So, there is no need to
    // set the indy_rf flag since any subsequent invokedynamic instruction which shares
    // this bootstrap method will encounter the resolution of MethodHandleInError.
    oop bsm_info = pool->resolve_bootstrap_specifier_at(pool_index, THREAD);
    Exceptions::wrap_dynamic_exception(CHECK);
    assert(bsm_info != NULL, "");
    // FIXME: Cache this once per BootstrapMethods entry, not once per CONSTANT_InvokeDynamic.
    bootstrap_specifier = Handle(THREAD, bsm_info);
  }
  if (!cpce->is_f1_null()) {
    methodHandle method(     THREAD, cpce->f1_as_method());
    Handle       appendix(   THREAD, cpce->appendix_if_resolved(pool));
    Handle       method_type(THREAD, cpce->method_type_if_resolved(pool));
    result.set_handle(method, appendix, method_type, THREAD);
    Exceptions::wrap_dynamic_exception(CHECK);
    return;
  }

  if (TraceMethodHandles) {
    ResourceMark rm(THREAD);
    tty->print_cr("resolve_invokedynamic #%d %s %s in %s",
                  ConstantPool::decode_invokedynamic_index(index),
                  method_name->as_C_string(), method_signature->as_C_string(),
                  current_klass->name()->as_C_string());
    tty->print("  BSM info: "); bootstrap_specifier->print();
  }

  resolve_dynamic_call(result, pool_index, bootstrap_specifier, method_name,
                       method_signature, current_klass, THREAD);
  if (HAS_PENDING_EXCEPTION && PENDING_EXCEPTION->is_a(SystemDictionary::LinkageError_klass())) {
    int encoded_index = ResolutionErrorTable::encode_cpcache_index(index);
    bool recorded_res_status = cpce->save_and_throw_indy_exc(pool, pool_index,
                                                             encoded_index,
                                                             pool()->tag_at(pool_index),
                                                             CHECK);
    if (!recorded_res_status) {
      // Another thread got here just before we did.  So, either use the method
      // that it resolved or throw the LinkageError exception that it threw.
      if (!cpce->is_f1_null()) {
        methodHandle method(     THREAD, cpce->f1_as_method());
        Handle       appendix(   THREAD, cpce->appendix_if_resolved(pool));
        Handle       method_type(THREAD, cpce->method_type_if_resolved(pool));
        result.set_handle(method, appendix, method_type, THREAD);
        Exceptions::wrap_dynamic_exception(CHECK);
      } else {
        assert(cpce->indy_resolution_failed(), "Resolution failure flag not set");
        ConstantPool::throw_resolution_error(pool, encoded_index, CHECK);
      }
      return;
    }
    assert(cpce->indy_resolution_failed(), "Resolution failure flag wasn't set");
  }
}

LinkResolver::resolve_invokedynamic用pool->resolve_bootstrap_specifier_at找到bsm（BootStrap Method）方法，然后resolve_dynamic_call根据bsm方法找到调用方法。其中找bsm会call到Java代码MethodHandleNatives.linkMethodHandleConstant，调用bsm也会call到Java代码的MethodHandleNatives.linkCallSite。

换句话说，prepare_invoke最终调用MethodHandleNatives.linkMethodHandleConstant找bsm方法，然后调用MethodHandleNatives.linkCallSite根据bsm方法找到最终调用的方法，然后jump_from_interpreted调用这个方法。

Java层

java层就好很多了，假设我们的代码是

public class MHTest {
    public static void main(String[] args) {
        Runnable r = ()->{
            System.out.println("fk");
        };
        r.run();
    }
}

脑补一下，jvm走到lambda处会调用模板解释器解释执行invokedynamic，这一步对应TemplateTable::invokedynamic。，然后前面提到这个方法先用linkMethodHandleConstant找bsm方法：

然后它返回一个bsm方法（MethodHandle），传递给linkCallSite作为bootstrapMethodObj，根据它找到要调用的方法：

与例子结合，lambda的bsm方法是LambdaMetafactory.metafacotry，linkCallSite会调用metafactory生成callsite：

然后返回metafactory找到的方法，由解释器调用它。再精简一点，invokedynamic本质上是增加了一个bsm方法作为中间层，用户可以写自己的bsm方法逻辑来确定调用哪个方法，以前的查找调用方法逻辑都是写死在虚拟机中，现在多了一个中间层。

LambdaMetafactory

public static CallSite metafactory(MethodHandles.Lookup caller,
                                       String invokedName,
                                       MethodType invokedType,
                                       MethodType samMethodType,
                                       MethodHandle implMethod,
                                       MethodType instantiatedMethodType)
            throws LambdaConversionException {
        AbstractValidatingLambdaMetafactory mf;
        mf = new InnerClassLambdaMetafactory(caller, invokedType,
                                             invokedName, samMethodType,
                                             implMethod, instantiatedMethodType,
                                             false, EMPTY_CLASS_ARRAY, EMPTY_MT_ARRAY);
        mf.validateMetafactoryArgs();
        return mf.buildCallSite();
    }

实际上它调用的是InnerClassLambdaMetafactory.buildCallSite：

 CallSite buildCallSite() throws LambdaConversionException {
        // 1. 创建生成的类对象
        final Class<?> innerClass = spinInnerClass();
        if (invokedType.parameterCount() == 0) {
            // 2. 用反射获取构造函数
            final Constructor<?>[] ctrs = AccessController.doPrivileged(
                    new PrivilegedAction<Constructor<?>[]>() {
                @Override
                public Constructor<?>[] run() {
                    Constructor<?>[] ctrs = innerClass.getDeclaredConstructors();
                    if (ctrs.length == 1) {
                        // The lambda implementing inner class constructor is private, set
                        // it accessible (by us) before creating the constant sole instance
                        ctrs[0].setAccessible(true);
                    }
                    return ctrs;
                }
                    });
            if (ctrs.length != 1) {
                throw new LambdaConversionException("Expected one lambda constructor for "
                        + innerClass.getCanonicalName() + ", got " + ctrs.length);
            }

            try {
                // 3. 创建实例 
                Object inst = ctrs[0].newInstance();
                // 4. 根据实例和samBase(接口类型)生成MethodHandle
                // 5. 生成ConstantCallSite
                return new ConstantCallSite(MethodHandles.constant(samBase, inst));
            }
            catch (ReflectiveOperationException e) {
                throw new LambdaConversionException("Exception instantiating lambda object", e);
            }
        } else {
            try {
                UNSAFE.ensureClassInitialized(innerClass);
                return new ConstantCallSite(
                        MethodHandles.Lookup.IMPL_LOOKUP
                             .findStatic(innerClass, NAME_FACTORY, invokedType));
            }
            catch (ReflectiveOperationException e) {
                throw new LambdaConversionException("Exception finding constructor", e);
            }
        }
    }

它生成一个.class文件，虚拟机默认不会输出，需要下面设置VM option-Djdk.internal.lambda.dumpProxyClasses=.，Dump出虚拟机生成的类我得到的是：

import java.lang.invoke.LambdaForm.Hidden;

// $FF: synthetic class
final class MethodReference$$Lambda$1 implements Encode {
    private MethodReference$$Lambda$1() {
    }

    @Hidden
    public void encode(Derive var1) {
        ((Base)var1).encrypt();
    }
}

该类实现了传来的接口函数(动态类生成，spring警告)。回到buildCallSite()源码，它使用MethodHandles.constant(samBase, inst)创建MethdHandle，放到CallSite里面，完成整个LambdaMetafactory的工作。 MethodHandles.constant(samBase, inst)相当于一个总是返回inst的方法。

总结是最重要的，说到底，invokedynamic的思想是增加一层的思想，计算机科学中的银弹。它通过BSM第一次执行时生成callsite，后面调用callsite，而正常流程是直接调用callsite。