What is the java heap?

For most applications, the Java Heap (Java Heap) is the largest piece of memory managed by the Java virtual machine. The Java heap is a memory area shared by all threads and is created when the virtual machine starts. The only purpose of this memory area is to store object instances, and almost all object instances allocate memory here. This is described in the Java virtual machine specification as follows: All object instances and arrays must allocate memory on the pair. However, with the development of JIT compilers and the gradual maturity of escape analysis technology, on-stack allocation and scalar replacement optimization technologies will This will lead to some subtle changes, and all objects allocated on the heap will gradually become less "absolute".

The Java heap is the main area managed by the garbage collector, so it is often called the "GC heap" (Garbage Collected heap). From the perspective of memory recycling, since current collectors basically use generational collection algorithms, the Java heap can also be subdivided into: new generation and old generation; in more detail, there are Eden space, From Survivor space, To Survivor space space etc. From the perspective of memory allocation, the thread-shared Java heap may be divided into multiple thread-private allocation buffers. However, no matter how divided it is, it has nothing to do with the stored content. No matter which area, object instances are still stored. The purpose of further dividing is to better recycle memory or allocate memory faster.

According to the Java Virtual Machine Specification, the Java heap can be in physically discontinuous memory space, as long as it is logically continuous, just like our disk space. During implementation, it can be either fixed size or expandable, but currently mainstream virtual machines are all implemented as scalable. If there is no memory in the heap to complete the instance allocation and the heap cannot be expanded, an OutOfMemoryError exception will be thrown.

Summary

1. The java heap is the largest piece of memory managed by the java virtual machine

2. The java heap is used by all threads Shared

3. The main function of the java heap is to store object instances

4. The ava heap is the main area managed by the garbage collector

5. The collector basically uses Generational collection algorithm

6. The Java heap can be divided into the new generation and the old generation. More detailed, it can also be divided into Eden area, From Surivor area, and To Surivor area

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