官方参考
Garbage Collection Roots
A garbage collection root is an object that is accessible from outside the heap. The following reasons make an object a GC root:
System Class
Class loaded by bootstrap/system class loader. For example, everything from the rt.jar like java.util.* .
JNI Local
Local variable in native code, such as user defined JNI code or JVM internal code.
JNI Global
Global variable in native code, such as user defined JNI code or JVM internal code.
Thread Block
Object referred to from a currently active thread block.
Thread
A started, but not stopped, thread.
Busy Monitor
Everything that has called wait() or notify() or that is synchronized. For example, by calling synchronized(Object) or by entering a synchronized method. Static method means class, non-static method means object.
Java Local
Local variable. For example, input parameters or locally created objects of methods that are still in the stack of a thread.
Native Stack
In or out parameters in native code, such as user defined JNI code or JVM internal code. This is often the case as many methods have native parts and the objects handled as method parameters become GC roots. For example, parameters used for file/network I/O methods or reflection.
Finalizable
An object which is in a queue awaiting its finalizer to be run.
Unfinalized
An object which has a finalize method, but has not been finalized and is not yet on the finalizer queue.
Unreachable
An object which is unreachable from any other root, but has been marked as a root by MAT to retain objects which otherwise would not be included in the analysis.
Java Stack Frame
A Java stack frame, holding local variables. Only generated when the dump is parsed with the preference set to treat Java stack frames as objects.
Unknown
An object of unknown root type. Some dumps, such as IBM Portable Heap Dump files, do not have root information. For these dumps the MAT parser marks objects which are have no inbound references or are unreachable from any other root as roots of this type. This ensures that MAT retains all the objects in the dump
关键点
- 所有Java线程当前活跃的栈帧里指向GC堆里的对象的引用；换句话说，当前所有正在被调用的方法的引用类型的参数/局部变量/临时值。
- VM的一些静态数据结构里指向GC堆里的对象的引用，例如说HotSpot VM里的Universe里有很多这样的引用。
- JNI handles，包括global handles和local handles
- （看情况）所有当前被加载的Java类
- （看情况）Java类的引用类型静态变量
- （看情况）Java类的运行时常量池里的引用类型常量（String或Class类型）
- （看情况）String常量池（StringTable）里的引用
分代GC对GC roots的定义的影响
分代式GC是一种部分收集（partial collection）的做法。在执行部分收集时，从GC堆的非收集部分指向收集部分的引用，也必须作为GC roots的一部分。
具体到分两代的分代式GC来说，如果第0代叫做young gen，第1代叫做old gen，那么如果有minor GC / young GC只收集young gen里的垃圾，则young gen属于“收集部分”，而old gen属于“非收集部分”，那么从old gen指向young gen的引用就必须作为minor GC / young GC的GC roots的一部分。
继续具体到HotSpot VM里的分两代式GC来说，除了old gen到young gen的引用之外，有些带有弱引用语义的结构，例如说记录所有当前被加载的类的SystemDictionary、记录字符串常量引用的StringTable等，在young GC时必须要作为strong GC roots，而在收集整堆的full GC时则不会被看作strong GC roots。
换句话说，young GC比full GC的GC roots还要更大一些。如果不能理解这个道理，那整个讨论也就无从谈起了。
参考来源
唠叨一下
- 完全属于面试题，对编程没有什么卵用
