A garbage collection root is an object that is accessible fromoutside the heap. The following reasons make an object a GCroot:
- System Class
- Class loaded by bootstrap/system class loader. Forexample, everything from the rt.jar like java.util.* .
- JNI Local
- Local variable in native code, such as user definedJNI code or JVM internal code.
- JNI Global
- Global variable in native code, such as userdefined 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 orlocally created objects of methods that are still in the stack of athread.
- Native Stack
- In or out parameters in native code, such as userdefined JNI code or JVM internal code. This is often the case asmany methods have native parts and the objects handled as methodparameters become GCroots. For example, parameters used forfile/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 andis 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 notbe included in the analysis.
- Java Stack Frame
- A Java stack frame, holding local variables. Only generated when thedump 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 haveno 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.