Garbage collection roots

Garbage collection roots

A garbage collection root is an object that is accessible from outside the heap.

Memory Analyzer categorizes garbage collection roots according to the following list:

System class

A class that was loaded by the bootstrap loader, or the system class loader. For example, this category includes all classes in the rt.jar file (part of the Java runtime environment), such as those in the java.util.* package.

JNI local

A local variable in native code, for example user-defined JNI code or JVM internal code.

JNI global

A global variable in native code, for example user-defined JNI code or JVM internal code.

Thread block

An object that was referenced from an active thread block.

Thread

A running thread.

Busy monitor

Everything that called the wait() or notify() methods, or that is synchronized, for example by calling the synchronized(Object) method or by entering a synchronized method. If the method was static, the root is a class, otherwise it is an object.

Java local

A local variable. For example, input parameters, or locally created objects of methods that are still in the stack of a thread.

Native stack

Input or output parameters in native code, for example user-defined JNI code or JVM internal code. Many methods have native parts, and the objects that are handled as method parameters become garbage collection roots. For example, parameters used for file, network, I/O, or reflection operations.

Finalizer

An object that is in a queue, waiting for a finalizer to run.

Unfinalized

An object that has a finalize method, but was not finalized, and is not yet on the finalizer queue.

Unreachable

An object that is unreachable from any other root, but was marked as a root by Memory Analyzer so that the object can be included in an analysis.

Unreachable objects are often the result of optimizations in the garbage collection algorithm. For example, an object might be a candidate for garbage collection, but be so small that the garbage collection process would be too expensive. In this case, the object might not be garbage collected, and might remain as an unreachable object.

By default, unreachable objects are excluded when Memory Analyzer parses the heap dump. These objects are therefore not shown in the histogram, dominator tree, or query results. You can change this behavior by clicking File > Preferences... > IBM Diagnostic Tools for Java - Memory Analyzer, then selecting the Keep unreachable objects check box.

Java stack frame

A Java stack frame, which holds local variables. This type of garbage collection root is only generated if you set the Preferences to treat Java stack frames as objects. For more information, see Java Basics: Threads and thread stack queries.

Unknown

An object of unknown root type. Some dumps, such as IBM Portable Heap Dump (.phd) files, do not have root information. In this case, the Memory Analyzer parser marks objects that have no inbound references, or are unreachable from any other root, as unknown. This action ensures that Memory Analyzer retains all the objects in the dump.

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