Dalvik的指令运行是解释器+JIT的方式，解释器就是虚拟机来对Javac编译出来的字节码，做译码、运行，而不是转化成CPU的指令集。由CPU来做译码，运行。可想而知。解释器的效率是相对较低的，所以出现了JIT(Just In Time)，JIT是将运行次数较多的函数，做即时编译，在运行时刻，编译成本地目标代码。JIT能够看成是解释器的一个补充优化。再之后又出现了Art虚拟机的AOT(Ahead Of Time)模式，做静态编译，在Apk安装的时候就会做字节码的编译，从而效率直逼静态语言。
Java全部的方法都是类方法，因此Dalvik的字节码运行就两种。一是类的Method。包含静态和非静态。两者的差距也就是有没有this參数。二就是类的初始化代码，就是类载入的时候。成员变量的初始化以及显式的类初始化块代码。
当中类的初始化代码在dalvik/vm/oo/Class.cpp的dvmInitClass：
bool dvmInitClass(ClassObject* clazz)
{
...
dvmLockObject(self, (Object*) clazz);
...
android_atomic_release_store(CLASS_INITIALIZING,
(int32_t*)(void*)&clazz->status);
dvmUnlockObject(self, (Object*) clazz);
...
initSFields(clazz); /* Execute any static initialization code.
*/
method = dvmFindDirectMethodByDescriptor(clazz, "<clinit>", "()V");
if (method == NULL) {
LOGVV("No <clinit> found for %s", clazz->descriptor);
} else {
LOGVV("Invoking %s.<clinit>", clazz->descriptor);
JValue unused;
dvmCallMethod(self, method, NULL, &unused);
}
...
}
从代码可见。类初始化的主要代码逻辑包含：
类对象加锁。所以类的载入是单线程的
初始化static成员（initSFields）
调用<cinit>，静态初始化块
类的初始化块代码在<cinit>的成员函数里。可见Dalvik的字节码解释，本质上还是类成员函数的解释运行。
虚拟机以Method作为解释器的运行单元。其入口就统一为dvmCallMethod，该函数的定义在dalvik/vm/interp/Stack.cpp里。
void dvmCallMethod(Thread* self, const Method* method, Object* obj,
JValue* pResult, ...)
{
va_list args;
va_start(args, pResult);
dvmCallMethodV(self, method, obj, false, pResult, args);
va_end(args);
} void dvmCallMethodV(Thread* self, const Method* method, Object* obj,
bool fromJni, JValue* pResult, va_list args)
{
...
if (dvmIsNativeMethod(method)) {
TRACE_METHOD_ENTER(self, method);
/*
* Because we leave no space for local variables, "curFrame" points
* directly at the method arguments.
*/
(*method->nativeFunc)((u4*)self->interpSave.curFrame, pResult,
method, self);
TRACE_METHOD_EXIT(self, method);
} else {
dvmInterpret(self, method, pResult);
}
…
}
Java的Method有native函数和非native函数。native的函数的代码段是在so里。是本地指令集而非虚拟机的字节码。
虚拟机以Method作为解释器的运行单元，其入口就统一为dvmCallMethod，该函数的定义在dalvik/vm/interp/Stack.cpp里。
void dvmCallMethod(Thread* self, const Method* method, Object* obj,
JValue* pResult, ...)
{
va_list args;
va_start(args, pResult);
dvmCallMethodV(self, method, obj, false, pResult, args);
va_end(args);
} void dvmCallMethodV(Thread* self, const Method* method, Object* obj,
bool fromJni, JValue* pResult, va_list args)
{
...
if (dvmIsNativeMethod(method)) {
TRACE_METHOD_ENTER(self, method);
/*
* Because we leave no space for local variables, "curFrame" points
* directly at the method arguments.
*/
(*method->nativeFunc)((u4*)self->interpSave.curFrame, pResult,
method, self);
TRACE_METHOD_EXIT(self, method);
} else {
dvmInterpret(self, method, pResult);
}
…
}
假设method是个native的函数，那么就直接调用nativeFunc这个函数指针，否则就调用dvmInterpret代码，dvmInterpret就是解释器的入口。
假设把Dalvik函数运行的调用栈画出来。我们会更清楚整个流程。
public class HelloWorld {
public int foo(int i, int j){
int k = i + j;
return k;
}
public static void main(String[] args) {
System.out.print(new HelloWorld().foo(1, 2));
}
}
Dalvik虚拟机有两个栈，一个Java栈。一个是VM的native栈。vm的栈是OS的函数调用栈。Java的栈则是由VM管理的栈，每次在dvmCallMethod的时候，在Method运行之前，会调用dvmPushInterpFrame(java→java)或者dvmPushJNIFrame(java→native)。JNI的Frame比InterpFrame少了局部变量的栈空间，native函数的局部变量是在vm的native栈里，由OS负责压栈出栈。DvmCallMethod结束的时候会调用dvmPopFrame做Java
Stack的出栈。
所以Java Method的运行就是dvmInterpret函数对这个Method的字节码做解析，函数的实參与局部变量都在Java的Stack里获取。SaveBlock是StackSaveArea数据结构。里面包括了当前函数相应的栈信息，包括返回地址等。而Native Method的运行就是Method的nativeFunc的运行，实參和局部变量都是在VM的native stack里。
Method的nativeFunc是native函数的入口，dalvik虚拟机上的java 的函数hook技术，都是通过改变Method的属性，SET_METHOD_FLAG(method, ACC_NATIVE)，伪装成native函数。再设置nativeFunc作为钩子函数。从而实现hook功能。非常显然，hook了的method不再具有多态性。
nativeFunc的默认函数是dvmResolveNativeMethod(vm/Native.cpp)
void dvmResolveNativeMethod(const u4* args, JValue* pResult,
const Method* method, Thread* self)
{
ClassObject* clazz = method->clazz; /*
* If this is a static method, it could be called before the class
* has been initialized.
*/
if (dvmIsStaticMethod(method)) {
if (!dvmIsClassInitialized(clazz) && !dvmInitClass(clazz)) {
assert(dvmCheckException(dvmThreadSelf()));
return;
}
} else {
assert(dvmIsClassInitialized(clazz) ||
dvmIsClassInitializing(clazz));
} /* start with our internal-native methods */
DalvikNativeFunc infunc = dvmLookupInternalNativeMethod(method);
if (infunc != NULL) {
/* resolution always gets the same answer, so no race here */
IF_LOGVV() {
char* desc = dexProtoCopyMethodDescriptor(&method->prototype);
LOGVV("+++ resolved native %s.%s %s, invoking",
clazz->descriptor, method->name, desc);
free(desc);
}
if (dvmIsSynchronizedMethod(method)) {
ALOGE("ERROR: internal-native can't be declared 'synchronized'");
ALOGE("Failing on %s.%s", method->clazz->descriptor, method->name);
dvmAbort(); // harsh, but this is VM-internal problem
}
DalvikBridgeFunc dfunc = (DalvikBridgeFunc) infunc;
dvmSetNativeFunc((Method*) method, dfunc, NULL);
dfunc(args, pResult, method, self);
return;
} /* now scan any DLLs we have loaded for JNI signatures */
void* func = lookupSharedLibMethod(method);
if (func != NULL) {
/* found it, point it at the JNI bridge and then call it */
dvmUseJNIBridge((Method*) method, func);
(*method->nativeFunc)(args, pResult, method, self);
return;
} IF_ALOGW() {
char* desc = dexProtoCopyMethodDescriptor(&method->prototype);
ALOGW("No implementation found for native %s.%s:%s",
clazz->descriptor, method->name, desc);
free(desc);
} dvmThrowUnsatisfiedLinkError("Native method not found", method);
}
dvmResolveNativeMethod首先会调用dvmLookupInternalNativeMethod查询这个函数是否预置的函数，主要是查以下的函数集：
static DalvikNativeClass gDvmNativeMethodSet[] = {
{ "Ljava/lang/Object;", dvm_java_lang_Object, 0 },
{ "Ljava/lang/Class;", dvm_java_lang_Class, 0 },
{ "Ljava/lang/Double;", dvm_java_lang_Double, 0 },
{ "Ljava/lang/Float;", dvm_java_lang_Float, 0 },
{ "Ljava/lang/Math;", dvm_java_lang_Math, 0 },
{ "Ljava/lang/Runtime;", dvm_java_lang_Runtime, 0 },
{ "Ljava/lang/String;", dvm_java_lang_String, 0 },
{ "Ljava/lang/System;", dvm_java_lang_System, 0 },
{ "Ljava/lang/Throwable;", dvm_java_lang_Throwable, 0 },
{ "Ljava/lang/VMClassLoader;", dvm_java_lang_VMClassLoader, 0 },
{ "Ljava/lang/VMThread;", dvm_java_lang_VMThread, 0 },
{ "Ljava/lang/reflect/AccessibleObject;",
dvm_java_lang_reflect_AccessibleObject, 0 },
{ "Ljava/lang/reflect/Array;", dvm_java_lang_reflect_Array, 0 },
{ "Ljava/lang/reflect/Constructor;",
dvm_java_lang_reflect_Constructor, 0 },
{ "Ljava/lang/reflect/Field;", dvm_java_lang_reflect_Field, 0 },
{ "Ljava/lang/reflect/Method;", dvm_java_lang_reflect_Method, 0 },
{ "Ljava/lang/reflect/Proxy;", dvm_java_lang_reflect_Proxy, 0 },
{ "Ljava/util/concurrent/atomic/AtomicLong;",
dvm_java_util_concurrent_atomic_AtomicLong, 0 },
{ "Ldalvik/bytecode/OpcodeInfo;", dvm_dalvik_bytecode_OpcodeInfo, 0 },
{ "Ldalvik/system/VMDebug;", dvm_dalvik_system_VMDebug, 0 },
{ "Ldalvik/system/DexFile;", dvm_dalvik_system_DexFile, 0 },
{ "Ldalvik/system/VMRuntime;", dvm_dalvik_system_VMRuntime, 0 },
{ "Ldalvik/system/Zygote;", dvm_dalvik_system_Zygote, 0 },
{ "Ldalvik/system/VMStack;", dvm_dalvik_system_VMStack, 0 },
{ "Lorg/apache/harmony/dalvik/ddmc/DdmServer;",
dvm_org_apache_harmony_dalvik_ddmc_DdmServer, 0 },
{ "Lorg/apache/harmony/dalvik/ddmc/DdmVmInternal;",
dvm_org_apache_harmony_dalvik_ddmc_DdmVmInternal, 0 },
{ "Lorg/apache/harmony/dalvik/NativeTestTarget;",
dvm_org_apache_harmony_dalvik_NativeTestTarget, 0 },
{ "Lsun/misc/Unsafe;", dvm_sun_misc_Unsafe, 0 },
{ NULL, NULL, 0 },
};
不是内置的话，就会载入so库。查询相应的native函数，查询的规则就是我们熟知的了，com.xx.Helloworld.foobar相应com_xx_Helloworld_foobar。
要注意的是，这个函数并非nativeFunc。接下来的dvmUseJNIBridge调用里，dvmCallJNIMethod会作为nativeFunc。这个函数主要须要将之前提到的java stack frame里的ins实參，转译成jni的函数调用參数。xposed/dexposed就会自己设置自己的nativeFun自己接管native函数的运行。
dvmInterpret是解释器的代码入口，代码位置在interp/Interp.cpp
void dvmInterpret(Thread* self, const Method* method, JValue* pResult)
{
InterpSaveState interpSaveState;
ExecutionSubModes savedSubModes;
. . .
interpSaveState = self->interpSave;
self->interpSave.prev = &interpSaveState;
. . . self->interpSave.method = method;
self->interpSave.curFrame = (u4*) self->interpSave.curFrame;
self->interpSave.pc = method->insns;
. . .
typedef void (*Interpreter)(Thread*);
Interpreter stdInterp;
if (gDvm.executionMode == kExecutionModeInterpFast)
stdInterp = dvmMterpStd;
#if defined(WITH_JIT)
else if (gDvm.executionMode == kExecutionModeJit ||
gDvm.executionMode == kExecutionModeNcgO0 ||
gDvm.executionMode == kExecutionModeNcgO1)
stdInterp = dvmMterpStd;
#endif
else
stdInterp = dvmInterpretPortable; // Call the interpreter
(*stdInterp)(self);
*pResult = self->interpSave.retval; /* Restore interpreter state from previous activation */
self->interpSave = interpSaveState;
#if defined(WITH_JIT)
dvmJitCalleeRestore(calleeSave);
#endif
if (savedSubModes != kSubModeNormal) {
dvmEnableSubMode(self, savedSubModes);
}
}
Thread的一个非常重要的field就是interpSave，是InterpSaveState类型的，里面包括了当前函数。pc。当前栈帧等重要的变量，dvmInterpret一開始调用的时候就会初始化。
Dalvik解释器有两个。一个是dvmInterpretPortable。一个是 dvmMterpStd。两者的差别在于。前者是从c++实现，后者是汇编实现。
dvmInterpretPortable是在vm/mterp/out/InterpC-portable.cpp中定义
void dvmInterpretPortable(Thread* self)
{
. . .
DvmDex* methodClassDex; // curMethod->clazz->pDvmDex
JValue retval; /* core state */
const Method* curMethod; // method we're interpreting
const u2* pc; // program counter
u4* fp; // frame pointer
u2 inst; // current instruction
/* instruction decoding */
u4 ref; // 16 or 32-bit quantity fetched directly
u2 vsrc1, vsrc2, vdst; // usually used for register indexes
/* method call setup */
const Method* methodToCall;
bool methodCallRange; /* static computed goto table */
DEFINE_GOTO_TABLE(handlerTable);
/* copy state in */
curMethod = self->interpSave.method;
pc = self->interpSave.pc;
fp = self->interpSave.curFrame;
retval = self->interpSave.retval; methodClassDex = curMethod->clazz->pDvmDex; . . . FINISH(0); /* fetch and execute first instruction */
/*--- start of opcodes ---*/ /* File: c/OP_NOP.cpp */
HANDLE_OPCODE(OP_NOP)
FINISH(1);
OP_END /* File: c/OP_MOVE.cpp */
HANDLE_OPCODE(OP_MOVE /*vA, vB*/)
vdst = INST_A(inst);
vsrc1 = INST_B(inst);
ILOGV("|move%s v%d,v%d %s(v%d=0x%08x)",
(INST_INST(inst) == OP_MOVE) ? "" : "-object", vdst, vsrc1,
kSpacing, vdst, GET_REGISTER(vsrc1));
SET_REGISTER(vdst, GET_REGISTER(vsrc1));
FINISH(1);
OP_END
…..
}
解释器的指令运行是通过跳转表来实现，DEFINE_GOTO_TABLE(handlerTable)定义了指令Op的goto表。
FINISH(0)，则表示从第一条指令開始运行，
# define FINISH(_offset) { \
ADJUST_PC(_offset); \
inst = FETCH(0); \
if (self->interpBreak.ctl.subMode) { \
dvmCheckBefore(pc, fp, self); \
} \
goto *handlerTable[INST_INST(inst)]; \
}
#define FETCH(_offset) (pc[(_offset)])
FETCH(0)获得当前要运行的指令，通过查跳转表handlerTable来跳转到这条指令的运行点，就是函数后面的HANDLE_OPCODE的定义。
后者是针对不同平台做过优化的解释器。
dvmMterpStd会做汇编级的优化，dvmMterpStdRun的入口就是针对不同的平台指令集，有相应的解释器代码，比方armv7 neon相应的代码就在mterp/out/InterpAsm-armv7-a-neon.S。
dvmMterpStdRun:
#define MTERP_ENTRY1 \
.save {r4-r10,fp,lr}; \
stmfd sp!, {r4-r10,fp,lr} @ save 9 regs
#define MTERP_ENTRY2 \
.pad #4; \
sub sp, sp, #4 @ align 64 .fnstart
MTERP_ENTRY1
MTERP_ENTRY2 /* save stack pointer, add magic word for debuggerd */
str sp, [r0, #offThread_bailPtr] @ save SP for eventual return /* set up "named" registers, figure out entry point */
mov rSELF, r0 @ set rSELF
LOAD_PC_FP_FROM_SELF() @ load rPC and rFP from "thread"
ldr rIBASE, [rSELF, #offThread_curHandlerTable] @ set rIBASE
. . .
/* start executing the instruction at rPC */
FETCH_INST() @ load rINST from rPC
GET_INST_OPCODE(ip) @ extract opcode from rINST
GOTO_OPCODE(ip) @ jump to next instruction
. . . #define rPC r4
#define rFP r5
#define rSELF r6
#define rINST r7
#define rIBASE r8
非jit的情况下，先是FETCH_INST把pc的指令载入到rINST寄存器，之后GET_INST_OPCODE获得操作码 and _reg, rINST, #255。是把rINST的低16位给ip寄存器，GOTO_OPCODE跳转到相应的地址。
#define GOTO_OPCODE(_reg) add pc, rIBASE, _reg, lsl #6
rIBASE 指向的curHandlerTable是跳转表的首地址。GOTO_OPCODE(ip)就将pc的地址指向该指令相应的操作码所在的跳转表地址。
static Thread* allocThread(int interpStackSize)
#ifndef DVM_NO_ASM_INTERP
thread->mainHandlerTable = dvmAsmInstructionStart;
thread->altHandlerTable = dvmAsmAltInstructionStart;
thread->interpBreak.ctl.curHandlerTable = thread->mainHandlerTable;
#endif
可见dvmAsmInstructionStart就是跳转表的入口，定义在dvmMterpStdRun里，
你能够在这里找到全部的Java字节码的指令相应的解释器代码。
比方new操作符相应的代码例如以下，先载入Thread.interpSave.methodClassDex，这是一个DvmDex指针，随后载入 DvmDex的pResClasses来查找类是否载入过。假设没载入过，那么跳转到 LOP_NEW_INSTANCE_resolve去载入类，假设载入过，就是类的初始化以及AllocObject的处理。LOP_NEW_INSTANCE_resolve就是调用clazz的dvmResolveClass载入。
/* ------------------------------ */
.balign 64
.L_OP_NEW_INSTANCE: /* 0x22 */
/* File: armv5te/OP_NEW_INSTANCE.S */
/*
* Create a new instance of a class.
*/
/* new-instance vAA, class@BBBB */
ldr r3, [rSELF, #offThread_methodClassDex] @ r3<- pDvmDex
FETCH(r1, 1) @ r1<- BBBB
ldr r3, [r3, #offDvmDex_pResClasses] @ r3<- pDvmDex->pResClasses
ldr r0, [r3, r1, lsl #2] @ r0<- resolved class
#if defined(WITH_JIT)
add r10, r3, r1, lsl #2 @ r10<- &resolved_class
#endif
EXPORT_PC() @ req'd for init, resolve, alloc
cmp r0, #0 @ already resolved?
beq .LOP_NEW_INSTANCE_resolve @ no, resolve it now
.LOP_NEW_INSTANCE_resolved: @ r0=class
ldrb r1, [r0, #offClassObject_status] @ r1<- ClassStatus enum
cmp r1, #CLASS_INITIALIZED @ has class been initialized?
bne .LOP_NEW_INSTANCE_needinit @ no, init class now
.LOP_NEW_INSTANCE_initialized: @ r0=class
mov r1, #ALLOC_DONT_TRACK @ flags for alloc call
bl dvmAllocObject @ r0<- new object
b .LOP_NEW_INSTANCE_finish @ continue .LOP_NEW_INSTANCE_needinit:
mov r9, r0 @ save r0
bl dvmInitClass @ initialize class
cmp r0, #0 @ check boolean result
mov r0, r9 @ restore r0
bne .LOP_NEW_INSTANCE_initialized @ success, continue
b common_exceptionThrown @ failed, deal with init exception /*
* Resolution required. This is the least-likely path.
*
* r1 holds BBBB
*/
.LOP_NEW_INSTANCE_resolve:
ldr r3, [rSELF, #offThread_method] @ r3<- self->method
mov r2, #0 @ r2<- false
ldr r0, [r3, #offMethod_clazz] @ r0<- method->clazz
bl dvmResolveClass @ r0<- resolved ClassObject ptr
cmp r0, #0 @ got null?
bne .LOP_NEW_INSTANCE_resolved @ no, continue
b common_exceptionThrown @ yes, handle exception
作者简单介绍： 田力。网易彩票Android端创始人，小米视频创始人。现任roobo技术经理、视频云技术总监 欢迎关注微信公众号 磨剑石，定期推送技术心得以及源代码分析等文章，谢谢