Share a quick way to load a Dex file

There is a Dexclassloader class on the Android system that can dynamically load DEX files, but there is a flaw in this class, which is that the first time you start and load a Dex file, you perform a dex2oat or dexopt operation, which normally doesn't feel like it's deficient, However, if you use it for reinforcement, you will have a very long initial startup time problem, for which you can use the following methods to increase the speed of the first boot:
The first is art mode, art mode to support the interpretation of the implementation of Dex file, do not need to compile, and do not need oat file, so direct hook execv function, so that the process of invoking dex2oat directly exit the good,
The hook callback function code is as follows:
int hook_execv (const char *name, char **argv) {
int ret = 0;
Char tmp[512];
if (! Isdex2oat (name)) {
Return Ori_execv (name, argv);
}
Exit (0);
}
Hook Way is got hook, the code is as follows:
void* Gothook (Soinfo *si, Char *target_name, void* hook_callback) {
Elf32_rel *rel = (elf32_rel*) si->plt_rela;
int count = si->plt_rela_count;
void *target_addr = 0;

int idx = 0;
for (idx = 0; idx<count; ++idx, ++rel) {
unsigned type = Elf32_r_type (Rel->r_info);
unsigned sym = Elf32_r_sym (Rel->r_info);

Elf32_addr reloc = (elf32_addr) (Rel->r_offset + (U4) si->base);
Elf32_addr sym_addr = 0;
Const char* sym_name = 0;


if (type = = 0) {
Continue
}

Elf32_sym *s = 0;
struct Soinfo_mine *lsi = 0;

if (sym!= 0) {
Sym_name = get_string ((elf32_sym*) si->symtab) [Sym].st_name, si];

printf ("%d sym_name=%s\n", IDX, sym_name);

if (strcmp (Sym_name, target_name))
Continue

printf ("%d sym_name=%s\n", IDX, sym_name);

TARGET_ADDR = * (void**) reloc;
if (hook_callback!= 0) {
Elf32_addr Seg_page_start = Page_start (reloc);
Elf32_addr seg_page_end = page_end (reloc + 4);
errno=0;
int ret = Mprotect ((void*) Seg_page_start, Seg_page_end-seg_page_start,
Prot_read | Prot_write);

* (void**) reloc = Hook_callback;
}
}
}


return target_addr;
}

int Jni_onload () {
...
Libart = (soinfo*) dlopen ("libart.so", Rtld_now);
ORI_EXECV = (FNEXECV) gothook ((soinfo*) Libart, "Execv", (void*) hook_execv);
...
}

The structure used in it can be taken from the Android linker source code.