System.gc初步分析
???? openjdk中的垃圾回收是一个庞大的课题，如何标记活动的对象，就涉及到了堆栈的、常量池的跟踪标记等，非常复杂。在此先做一个初步的分析。
???? 先从System.java中的gc方法开始阅读。
???? public static void gc() {
??????? Runtime.getRuntime().gc();
???? }
???? 跟踪进去，其实调用到了jvm.cpp里面的JVM_GC(void)
??? {
????????????? if (!DisableExplicitGC) { //如果可以直接进行垃圾回收，则执行下面的一步， -XX:+DisableExplicitGC?选项可以关闭jvm的直接垃圾回收
???????????????????? Universe::heap()->collect(GCCause::_java_lang_system_gc);
????????????? }
???? }
???? 垃圾回收的方式有多种，下面跟踪并发收集(ParallelScavengeHeap.cpp)。
???? void ParallelScavengeHeap::collect(GCCause::Cause cause) {
???????????? ...........
?????????????VM_ParallelGCSystemGC op(gc_count, full_gc_count, cause);
?????????????VMThread::execute(&op);
???? }
?????上面的exceute()方法，主要执行的是VM_ParallelGCSystemGC 的doit()方法，下面看看
???? void VM_ParallelGCSystemGC::doit() {
???????????? //在这主要看一下全部回收的代码
?????????????ParallelScavengeHeap* heap = (ParallelScavengeHeap*)Universe::heap();
?????????????heap->invoke_full_gc(false);
???? }
?????下面是invoke_full_gc代码
???? inline void ParallelScavengeHeap::invoke_full_gc(bool maximum_compaction)
??? {
???????????? .......
???????????? PSMarkSweep::invoke(maximum_compaction);//从名字看出这个方法主要做标记、清除操作
???? }
???? 跟踪进入上面方法
???? void PSMarkSweep::invoke(bool maximum_heap_compaction) {
??????????? .........
??????????? PSMarkSweep::invoke_no_policy(maximum_heap_compaction);
???? }
???? 上面invoke_no_policy非常复杂，在这仅对其第一阶段的操作进行简单分析。
???? void PSMarkSweep::mark_sweep_phase1(bool clear_all_softrefs) {
??????????? .........
??????????? //从根开始对强引用开始进行标记操作
????????????Universe::oops_do(mark_and_push_closure());
????????????ReferenceProcessor::oops_do(mark_and_push_closure());
??????????? JNIHandles::oops_do(mark_and_push_closure());?? // Global (strong) JNI handles
??????????? //跟踪每个线程堆栈，对堆栈里面的对象引用进行跟踪
??????????? Threads::oops_do(mark_and_push_closure());
??????????? ObjectSynchronizer::oops_do(mark_and_push_closure());
??????????? FlatProfiler::oops_do(mark_and_push_closure());
??????????? Management::oops_do(mark_and_push_closure());
??????????? JvmtiExport::oops_do(mark_and_push_closure());
????????????//跟踪常量池用到的每个类
??????????? SystemDictionary::always_strong_oops_do(mark_and_push_closure());
??????????? vmSymbols::oops_do(mark_and_push_closure());
???? }
???? 简单提一下，上面用到的MarkAndPushClosure，它其实就是将对象打上标志，并将对象指针压入一个栈中。采用的是回调函数用法，由上面的oops_do调用。代码如下
???? {
????????????if (!oopDesc::is_null(heap_oop)) {
?????????????????? oop obj = oopDesc::decode_heap_oop_not_null(heap_oop);
????????????????? if (!obj->mark()->is_marked()) {
????????????????????????? mark_object(obj);? //对遍历到的对象进行标志
???????????????????????? _marking_stack->push(obj);?//对象入栈
???????????????? }
??????????? }
?????}
?????好，回到前面，简单分析一下Threads::oops_do(mark_and_push_closure())。
???? void Threads::oops_do(OopClosure* f) {
?????????? for (JavaThread*?p = _thread_list; p;?p = X->next()){ //遍历所有用户线程
?????????????????? p->oops_do(f);
????????? }
????????? VMThread::vm_thread()->oops_do(f); //遍历vmThread
???? }
???? java对象的方法调用，其在方法内创建的局部变量都在线程堆栈内。javaThread::oops_do应该会对堆栈内的对象进行标记，下面看一下代码。
??? void JavaThread::oops_do(OopClosure* f) {
???????????Thread::oops_do(f);
?????????? ........
?????????? //下面是堆栈桢的遍历
???????????for(StackFrameStream fst(this); !fst.is_done(); fst.next()) {
?????????????????? fst.current()->oops_do(f, fst.register_map());
?????????? }
?????????? ........
??? }
????void? Frame::oops_do(OopClosure* f, RegisterMap* map) {
?????????? oops_do_internal(f, map, true);
??? }
??? void frame::oops_do_internal(OopClosure* f, RegisterMap* map, bool use_interpreter_oop_map_cache) {
?????????? if (is_interpreted_frame())??? { //以解释桢为例
?????????????? oops_interpreted_do(f, map, use_interpreter_oop_map_cache);
??????????? }
?? }
???void frame::oops_interpreted_do(OopClosure* f, const RegisterMap* map, bool query_oop_map_cache) {
??????????............
?????????? oops_interpreted_locals_do(f, max_locals, mask);
??? oops_interpreted_expressions_do(f, signature, is_static,
??????????????????????????????????? m->max_stack(),
??????????????????????????????????? max_locals, mask);
?????????............
? }
??void frame::oops_interpreted_locals_do(OopClosure *f,
????????????????????????????????????? int max_locals,
????????????????????????????????????? InterpreterOopMap *mask) {
????????? for (int i = 0; i < max_locals; i++ ) {
????????????????? Tag tag = interpreter_frame_local_tag(i);
????????????????? if (tag == TagReference) { //如果是对象引用，则进行标记，总算找到了
????????????????????????? oop* addr = (oop*) interpreter_frame_local_at(i);
????????????????????????? assert((intptr_t*)addr >= sp(), "must be inside the frame");
??????????????????????????f->do_oop(addr);
??????? ??? }
?}
?堆栈里面存着基本类型及引用类型，它们的位置及垃圾回收算法是如何区分这些类型的，这个还需要进一步的分析。
