Perm Gen is populated by JVM at runtime based on the classes used by the application. Permanent Generation or “Perm Gen” contains the application metadata required by the JVM to describe the classes and methods used in the application. Java Memory Models Permanent Generation (Replaced by Metaspace since Java 8) This prevents objects from being promoted just because they were allocated right before a young collection is started. The keep area contains the most recently allocated objects in the nursery and is not garbage collected until the next young generation. Recent releases include a part of nursery called as keep area and it is reserved. Typically, a young collection frees a given amount of memory much faster than an old collection or a garbage collection of a single-generational heap (a heap without a nursery). A young collection is designed to be swift at finding newly allocated objects that are still alive and moving them away from the nursery. The reasoning behind a nursery is that most objects are temporary and short-lived. Old generation garbage collection is called as Major GC and usually takes longer. Usually, garbage collection is performed in Old generation memory when it’s full. Old generation memory contains the objects that are long-lived and survived after many rounds of Minor GC. When the old generation becomes full, garbage is collected there and the process is called as old collection. Usually, it is done by setting a threshold for the age of the nursery objects before they become eligible to promote to the old generation Objects that have survived many cycles of GC, are moved to the old generation memory space.So at a time, one of the survivor space is always empty Minor GC also checks the survivor objects and moves them to the other survivor space.When Eden space is filled with objects, Minor GC is performed and all the survivor objects are moved to one of the survivor spaces.Most of the newly created objects are located in the Eden Memory space.Important points about the nursery space: The nursery is divided into three parts – Eden Memory and two Survivor Memory spaces. This garbage collection is called Minor GC. When the nursery becomes full, garbage is collected by running a special young collection, where all the objects that have lived long enough in the nursery are promoted (moved) to the old space, thus freeing up the nursery for more object allocation. The nursery is a part of the heap reserved for the allocation of new objects. The JVM heap is physically divided into two parts (or generations): nursery (or young space/young generation) and old space (or old generation). Java Virtual Machine uses this space to store the JVM code itself, JVM internal structures, loaded profiler agent code, and data, etc. This can be changed using –XX:MaxPermSize VM option. The default maximum size of non-heap memory is 64 MB. 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 Java Virtual Machine has memory other than the heap, referred to as Non-Heap Memory. By default, the maximum heap size is set to 64 MB. Maximum heap size can be set using –Xmx option. The heap can be of fixed size or variable size depending on the garbage collection strategy. The size of the heap can be specified using –Xms VM option. The heap is created when the Java Virtual Machine starts up and may increase or decrease in size while the application runs. Heap memory is the run time data area from which the memory for all java class instances and arrays is allocated. JVM memory is divided into multiple parts: Heap Memory, Non-Heap Memory, and Other. The Java Virtual Machine loads the code, verifies the code, executes the code, manages memory (this includes allocating memory from the Operating System (OS), managing Java allocation including heap compaction and removal of garbage objects) and finally provides the runtime environment. But the implementation has been provided by Sun and other companies), implementation (known as (JRE) Java Runtime Environment) and instance (after writing Java command, to run Java class, an instance of JVM is created). There are three notions of JVM: specification (where working of JVM is specified. The JVM is an abstract computing machine that enables a computer to run a Java program.
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