Android培训班(67)dex文件打开流程

从上面可知调用函数Dalvik_dalvik_system_DexFile_openDexFile来打开Ｄex文件，这个函数的源码如下：staticvoid Dalvik_dalvik_system_DexFile_openDexFile(const u4* args, JValue* pResult){ StringObject* sourceNameObj =(StringObject*) args[0];这行是输入的Jar或Ｄex文件名参数。
StringObject* outputNameObj =(StringObject*) args[1];这行是输出的文件名参数。
int flags = args[2];这行是处理的标示。
DexOrJar* pDexOrJar = NULL; JarFile* pJarFile; RawDexFile* pRawDexFile; char* sourceName; char* outputName;
if (sourceNameObj == NULL) { dvmThrowException("Ljava/lang/NullPointerException;",NULL); RETURN_VOID(); }这段代码是当输入文件名称为空对象时，就抛出异常。

sourceName =dvmCreateCstrFromString(sourceNameObj);这行代码调用函数dvmCreateCstrFromString把java字符串转换Ｃ字符串，由于Ｊava对象表示的字符串并不能立即就使用到Ｃ语言里，所以需要转换才能使用。
if (outputNameObj != NULL) outputName =dvmCreateCstrFromString(outputNameObj); else outputName = NULL;这段代码是同样把输出字符串转换Ｃ字符串。

/* * We have to deal with the possibility thatsomebody might try to * open one of our bootstrap class DEXfiles. The set of dependencies * will be different, and hence the resultsof optimization might be * different, which means we'd actually needto have two versions of * the optimized DEX: one that only knowsabout part of the boot class * path, and one that knows about everythingin it. The latter might * optimize field/method accesses based on aclass that appeared later * in the class path. * * We can't let the user-defined classloader open it and start using * the classes, since the optimized form ofthe code skips some of * the method and field resolution that wewould ordinarily do, and * we'd have the wrong semantics. * * We have to reject attempts to manuallyopen a DEX file from the boot * class path. The easiest way to do thisis by filename, which works * out because variations in name (e.g."/system/framework/./ext.jar") * result in us hitting a differentdalvik-cache entry. It's also fine * if the caller specifies their own outputfile. */ if(dvmClassPathContains(gDvm.bootClassPath, sourceName)) { LOGW("Refusing to reopen boot DEX'%s'\n", sourceName); dvmThrowException("Ljava/io/IOException;", "Re-opening BOOTCLASSPATH DEXfiles is not allowed"); free(sourceName); RETURN_VOID(); }这段代码是判断用户是否加载系统目录下面的Ｄex文件，如果加载就要拒绝这样的操作，因为系统启动时已经加载了一份这样的优化代码，没有必要再次加载一次。

/* * Try to open it directly as a DEX. Ifthat fails, try it as a Zip * with a "classes.dex" inside. */ if (dvmRawDexFileOpen(sourceName,outputName, &pRawDexFile, false) == 0) { LOGV("Opening DEX file '%s'(DEX)\n", sourceName);
pDexOrJar = (DexOrJar*)malloc(sizeof(DexOrJar)); pDexOrJar->isDex = true; pDexOrJar->pRawDexFile = pRawDexFile;这段代码是尝试加载Ｄex文件，但基本不存在直接加Ｄex文件的情况，因此在函数dvmRawDexFileOpen还是空函数，没有实际的内容。

}else if (dvmJarFileOpen(sourceName, outputName, &pJarFile, false)== 0) { LOGV("Opening DEX file '%s'(Jar)\n", sourceName);
pDexOrJar = (DexOrJar*)malloc(sizeof(DexOrJar)); pDexOrJar->isDex = false; pDexOrJar->pJarFile = pJarFile;这段代码是加载Jar文件，就是从这里加载Dex文件到缓存里。
}else { LOGV("Unable to open DEX file'%s'\n", sourceName); dvmThrowException("Ljava/io/IOException;","unable to open DEX file"); }

if (pDexOrJar != NULL) { pDexOrJar->fileName = sourceName;这行代码保存文件名称到Dex文件对象里。
/* add to hash table */ u4 hash = dvmComputeUtf8Hash(sourceName);这行代码通过文件名称计算ＨＡＳＨ串，加速对文件的查找速度。
void* result; dvmHashTableLock(gDvm.userDexFiles); result =dvmHashTableLookup(gDvm.userDexFiles, hash, pDexOrJar, hashcmpDexOrJar, true); dvmHashTableUnlock(gDvm.userDexFiles);这段代码添加ＨＡＳＨ表里，以便后面查找使用。
if (result != pDexOrJar) { LOGE("Pointer has already beenadded?\n"); dvmAbort(); }
pDexOrJar->okayToFree = true; }else free(sourceName);
RETURN_PTR(pDexOrJar);这行代码返回打开的文件对象。
}
这个函数是通过J***A调用时输入Ｄex文件名称，然后加载Ｄex文件，最后把这个文件名称放到ＨＡＳＨ表里，然后返回打开的对象。