深入浅出 System.gc() 源码解读

/**
     * Runs the garbage collector.
     * <p>
     * Calling the <code>gc</code> method suggests that the Java Virtual
     * Machine expend effort toward recycling unused objects in order to
     * make the memory they currently occupy available for quick reuse.
     * When control returns from the method call, the Java Virtual
     * Machine has made a best effort to reclaim space from all discarded
     * objects.
     * <p>
     * The call <code>System.gc()</code> is effectively equivalent to the
     * call:
     * <blockquote><pre>
     * Runtime.getRuntime().gc()
     * </pre></blockquote>
     *
     * @see     java.lang.Runtime#gc()
     */
    public static void gc() {
        Runtime.getRuntime().gc();
    }


/**
     * Runs the garbage collector.
     * Calling this method suggests that the Java virtual machine expend
     * effort toward recycling unused objects in order to make the memory
     * they currently occupy available for quick reuse. When control
     * returns from the method call, the virtual machine has made
     * its best effort to recycle all discarded objects.
     * <p>
     * The name <code>gc</code> stands for "garbage
     * collector". The virtual machine performs this recycling
     * process automatically as needed, in a separate thread, even if the
     * <code>gc</code> method is not invoked explicitly.
     * <p>
     * The method {@link System#gc()} is the conventional and convenient
     * means of invoking this method.
     */
    public native void gc();
----------------------------


JNIEXPORT void JNICALL
Java_java_lang_Runtime_gc(JNIEnv *env, jobject this)
{
    JVM_GC();
}


JVM_ENTRY_NO_ENV(void, JVM_GC(void))
  JVMWrapper("JVM_GC");
  if (!DisableExplicitGC) {
    Universe::heap()->collect(GCCause::_java_lang_system_gc);
  }
JVM_END


void GenCollectedHeap::collect(GCCause::Cause cause) {
  if (should_do_concurrent_full_gc(cause)) {
#ifndef SERIALGC
    // mostly concurrent full collection
    collect_mostly_concurrent(cause);
#else  // SERIALGC
    ShouldNotReachHere();
#endif // SERIALGC
  } else {
#ifdef ASSERT
    if (cause == GCCause::_scavenge_alot) {
      // minor collection only
      collect(cause, 0);
    } else {
      // Stop-the-world full collection
      collect(cause, n_gens() - 1);
    }
#else
    // Stop-the-world full collection
    collect(cause, n_gens() - 1);
#endif
  }
}


bool GenCollectedHeap::should_do_concurrent_full_gc(GCCause::Cause cause) {
  return UseConcMarkSweepGC &&
         ((cause == GCCause::_gc_locker && GCLockerInvokesConcurrent) ||
          (cause == GCCause::_java_lang_system_gc && ExplicitGCInvokesConcurrent));
}


void GenCollectedHeap::collect_mostly_concurrent(GCCause::Cause cause) {
  assert(!Heap_lock->owned_by_self(), "Should not own Heap_lock");

  MutexLocker ml(Heap_lock);
  // Read the GC counts while holding the Heap_lock
  unsigned int full_gc_count_before = total_full_collections();
  unsigned int gc_count_before      = total_collections();
  {
    MutexUnlocker mu(Heap_lock);
    VM_GenCollectFullConcurrent op(gc_count_before, full_gc_count_before, cause);
    VMThread::execute(&op);
  }
}


void CMSCollector::request_full_gc(unsigned int full_gc_count, GCCause::Cause cause) {
  GenCollectedHeap* gch = GenCollectedHeap::heap();
  unsigned int gc_count = gch->total_full_collections();
  if (gc_count == full_gc_count) {
    MutexLockerEx y(CGC_lock, Mutex::_no_safepoint_check_flag);
    _full_gc_requested = true;
    _full_gc_cause = cause;
    CGC_lock->notify();   // nudge CMS thread
  } else {
    assert(gc_count > full_gc_count, "Error: causal loop");
  }
}


void GenCollectedHeap::collect_locked(GCCause::Cause cause, int max_level) {
  if (_preloading_shared_classes) {
    report_out_of_shared_space(SharedPermGen);
  }
  // Read the GC count while holding the Heap_lock
  unsigned int gc_count_before      = total_collections();
  unsigned int full_gc_count_before = total_full_collections();
  {
    MutexUnlocker mu(Heap_lock);  // give up heap lock, execute gets it back
    VM_GenCollectFull op(gc_count_before, full_gc_count_before,
                         cause, max_level);
    VMThread::execute(&op);
  }
}


void VM_GenCollectFull::doit() {
  SvcGCMarker sgcm(SvcGCMarker::FULL);

  GenCollectedHeap* gch = GenCollectedHeap::heap();
  GCCauseSetter gccs(gch, _gc_cause);
  gch->do_full_collection(gch->must_clear_all_soft_refs(), _max_level);
}