Java GC safepoint

问题:

When a garbage collector freezes the application threads before cleaning up unreferenced objects, all threads are required to be at a "safepoint" in their execution. I found a wealth of writing describing the concept of a safepoint, but very few examples. Where will a safepoint be placed within a typical Java method and why? And more importantly, where can a safepoint not occur?

回答1:

The exact definition and implementation of a safepoint changes from one VM implementation to another, but considering Hotspot VM, you can find a nice definition in: Safepoints in HotSpot JVM.

HotSpot glossary says:

A point during program execution at which all GC roots are known and all heap object contents are consistent. From a global point of view, all threads must block at a safepoint before the GC can run.

Typically, the safepoint is implemented by JVM injecting a safepoint check into a method, most call sites qualify as safepoints - when reaching the safepoint check, the thread will check if the safepoint is required (e.g. FullGC is scheduled), if yes, then the thread blocks. When all the threads in the VM block, you have reached the safepoint where all the the objects in the VM are perfectly reachable. Then, the VM operation that requested the safepoint is performed (e.g. a FullGC), after that the threads are resumed.

Check for the list of VM operations requiring a safepoint : Safety First: Safepoints.

You can study safepoint behavior in Hotspot by using -XX:+PrintSafepointStatistics –XX:PrintSafepointStatisticsCount=1.

回答2:

Unfortunately this is a poor defined field. The JVM places safepoints when it decides to, but there is no specifications to when. One version/update to the next of Java can be different. There are some cases like Unsafe.copyMemory() which doesn't have a safepoint, but you cannot be sure where safepoints will be placed.