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Welcome to the Java_Revised wiki! Java (JVM) Memory Structure JVM memory is divided into multiple parts: Heap Memory, Non-Heap Memory, and Other.

Heap memory

Heap memory is the run time data area from which the memory for all java class instances and arrays is allocated. The heap is created when the JVM starts up and may increase or decrease in size while the application runs. The size of the heap can be specified using –Xms VM option. The heap can be of fixed size or variable size depending on the garbage collection strategy. Maximum heap size can be set using –Xmx option. By default, the maximum heap size is set to 64 MB.

Non-Heap memory

The JVM has memory other than the heap, referred to as Non-Heap Memory. It is created at the JVM startup and stores per-class structures such as runtime constant pool, field and method data, and the code for methods and constructors, as well as interned Strings. The default maximum size of non-heap memory is 64 MB. This can be changed using –XX:MaxPermSize VM option.

Other memory

JVM uses this space to store the JVM code itself, JVM internal structures, loaded profiler agent code and data, etc.

JAVA Memory Model

The diagram below is the Java Memory Model for the Heap as well as the PermGen for any Java Application running in the Java Virtual Machine (JVM). The ratios are also provided to get a fair understanding of how the distribution of allowed memory is done across each of the generation types. All of the info is completely applicable up to Java 1.7 (inclusive). This diagram is also known as the 'Managed Area' of the memory model.

In addition to the above, there is a Stack Area, which can be configured using the -Xss option. This area holds the references on the heap, native references, pc registers, code cache, and local variables for all threads. This is also known as the 'Native Area' of the memory model.

Managed Area of the Java Memory Model

[Young Generation/Nursery] Eden Space

All new objects are first created in the Eden Space. As soon as it reaches an arbitrary threshold decided by the JVM, a minor garbage collection (Minor GC) kicks in. It first removes all the non-referenced objects and moves referenced objects from the 'eden' and 'from' into the 'to' survivor space. Once the GC is over, the 'from' and 'to' roles (names) are swapped. Points to remember

• Minor GC also checks the survivor objects and move them to the other survivor space. So at a time, one of the survivor space is always empty.
• Objects that are survived after many cycles of GC, are moved to the Old generation memory space. Usually it’s done by setting a threshold for the age of the young generation objects before they become eligible to promote to Old generation.

[Young Generation/Nursery] Survivor 1 (From)

This is a part of the survivor space (You may think of this a role in the survivor space). This was the 'to' role during the previous garbage collection (GC).

[Young Generation/Nursery] Suvrivor 2 (To)

This is also a part of the survivor space (You may think of this as a role in the survivor space too). It is here, during the GC, that all the referenced objects are moved to, from 'from' and 'eden'.

[Old Generation] Tenured

Depending on the threshold limits, which can be checked by using -XX:+PrintTenuringDistribution, which shows the objects (space in bytes) by age, objects are moved from the 'to' Survivor space to the Tenured space. 'Age' is the number of times that it has moved within the survivor space.

There are other important flags like, -XX:InitialTenuringThreshold, -XX:MaxTenuringThreshold and -XX:TargetSurvivorRatio which lead to an optimum utilization of the tenured as well as the survivor spaces.

By setting -XX:InitialTenuringThreshold and -XX:MaxTenuringThreshold we allow an initial value and a maximum value for 'Age' while maintaining the percentage utilization in the 'Survivor (To)' as specified by the -XX:+NeverTenure and -XX:+AlwaysTenure, which encouraged never to be used to tenure an object (risky to use). The opposite usage is to always tenure, which means to always use the 'old generation'.

The garbage collection that happens here is the major garbage collection (Major GC). This is usually triggered when the heap is full or the old generation is full. This is usually a 'Stop-the-World' event or thread that takes over to perform the garbage collection. There is another type of GC named full garbage collection (Full GC) which involves other memory areas such as the permgen space.

Other important and interesting flags related to the overall heap are -XX:SurvivorRatio and -XX:NewRatio, which specify the eden space to the survivor space ratio and old generation to the new generation ratio.

[Permanent Generation] Permgen space (Replaced by Metaspace since Java 8)

The 'Permgen' is used to store the following information: Constant Pool (Memory Pool), Field & Method Data and Code. Each of them related to the same specifics as their name suggests.