Java中可作为gc root 的对象有哪些？

1 、 虚拟机栈（栈帧中的本地变量表）中引用的对象。

2、 本地方法栈中JNI（即一般说的native方法）引用的对象。

3、 方法区中的静态变量和常量引用的对象。

以上摘自《深入理解java虚拟机》书中。

我的理解是 上面是从内存分区和虚拟机真实工作流程角度来说的。即虚拟机寻找 gc root 时会去这三个地方找。（有待验证）

 从程序的角度来说就是，找到一段程序运行的整个过程中始终会存活的对象，这些对象的特点是始终会存活，不会死亡。即一些静态变量 和常量所引用的对象等。

 

 

eclipse 文档

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.