garbage-collection - JNI 代码即与垃圾收集并发执行。

Question

它写在 http://psy-lob-saw.blogspot.com/2015/12/safepoints.html

A Java thread is at a safepoint while executing JNI code. Before crossing the native call boundary the stack is left in a consistent state before handing off to the native code. This means that the thread can still run while at a safepoint.

这怎么可能？毕竟，我可以将对象的引用传递给 JNI .
在 JNI 中，我可以在该对象中设置一个字段。

很明显是收不到的(我们有本地引用)。但是，它可以在完整的 gc 收集期间通过 GC 移动到老年代。
所以，我们有以下情况:

GC collector:                          | Thread executing JNI code
compact old generation                 | modify object fields that can be 
and move object from young generation  | moved now! A catastrophe. 
to old generation.                     |

JVM 如何处理？

Answer 1

几乎每个 JNI 调用都有一个安全点保护。每当您从 native 方法调用 JNI 函数时，线程都会从 in_native 切换。至 in_vm状态。此转换的一部分是安全点检查。

见 ThreadStateTransition::transition_from_native()其中调用 JavaThread::check_safepoint_and_suspend_for_native_trans(thread)

// Slow path when the native==>VM/Java barriers detect a safepoint is in
// progress or when _suspend_flags is non-zero.
// Current thread needs to self-suspend if there is a suspend request and/or
// block if a safepoint is in progress.

也就是说，在 GC 处于事件状态时调用 JNI 函数的线程将被挂起，直到 GC 完成。