[java] How is the java memory pool divided?

I’m currently monitoring a Java application with jconsole. The memory tab lets you choose between:

Heap Memory Usage
Non-Heap Memory Usage
Memory Pool “Eden Space”
Memory Pool “Survivor Space”
Memory Pool “Tenured Gen”
Memory Pool “Code Cache”
Memory Pool “Perm Gen”

What is the difference between them ?

With Java8, non heap region no more contains PermGen but Metaspace, which is a major change in Java8, supposed to get rid of out of memory errors with java as metaspace size can be increased depending on the space required by jvm for class data.

Java Heap Memory is part of memory allocated to JVM by Operating System.

Objects reside in an area called the heap. The heap is created when the JVM starts up and may increase or decrease in size while the application runs. When the heap becomes full, garbage is collected.

You can find more details about Eden Space, Survivor Space, Tenured Space and Permanent Generation in below SE question:

Young , Tenured and Perm generation

PermGen has been replaced with Metaspace since Java 8 release.

Regarding your queries:

1. Eden Space, Survivor Space, Tenured Space are part of heap memory
2. Metaspace and Code Cache are part of non-heap memory.

Codecache: The Java Virtual Machine (JVM) generates native code and stores it in a memory area called the codecache. The JVM generates native code for a variety of reasons, including for the dynamically generated interpreter loop, Java Native Interface (JNI) stubs, and for Java methods that are compiled into native code by the just-in-time (JIT) compiler. The JIT is by far the biggest user of the codecache.

The Heap is divided into young and old generations as follows :

Young Generation: It is a place where an object lived for a short period and it is divided into two spaces:

• Eden Space: When object created using new keyword memory allocated on this space.
• Survivor Space (S0 and S1): This is the pool which contains objects which have survived after minor java garbage collection from Eden space.

Old Generation: This pool basically contains tenured and virtual (reserved) space and will be holding those objects which survived after garbage collection from the Young Generation.

• Tenured Space: This memory pool contains objects which survived after multiple garbage collection means an object which survived after garbage collection from Survivor space.

Explanation

Let's imagine our application has just started.

So at this point all three of these spaces are empty (Eden, S0, S1).

Whenever a new object is created it is placed in the Eden space.

When the Eden space gets full then the garbage collection process (minor GC) will take place on the Eden space and any surviving objects are moved into S0.

Our application then continues running add new objects are created in the Eden space the next time that the garbage collection process runs it looks at everything in the Eden space and in S0 and any objects that survive get moved into S1.

PS: Based on the configuration that how much time object should survive in Survivor space, the object may also move back and forth to S0 and S1 and then reaching the threshold objects will be moved to old generation heap space.

The new keyword allocates memory on the Java heap. The heap is the main pool of memory, accessible to the whole of the application. If there is not enough memory available to allocate for that object, the JVM attempts to reclaim some memory from the heap with a garbage collection. If it still cannot obtain enough memory, an OutOfMemoryError is thrown, and the JVM exits.

The heap is split into several different sections, called generations. As objects survive more garbage collections, they are promoted into different generations. The older generations are not garbage collected as often. Because these objects have already proven to be longer lived, they are less likely to be garbage collected.

When objects are first constructed, they are allocated in the Eden Space. If they survive a garbage collection, they are promoted to Survivor Space, and should they live long enough there, they are allocated to the Tenured Generation. This generation is garbage collected much less frequently.

There is also a fourth generation, called the Permanent Generation, or PermGen. The objects that reside here are not eligible to be garbage collected, and usually contain an immutable state necessary for the JVM to run, such as class definitions and the String constant pool. Note that the PermGen space is planned to be removed from Java 8, and will be replaced with a new space called Metaspace, which will be held in native memory. reference:http://www.programcreek.com/2013/04/jvm-run-time-data-areas/