Overview

The heap is the largest memory space in the Java Virtual Machine (JVM) and is shared among all threads. It holds most of the objects and arrays created by the application. The heap is divided into the following regions:

• Young generation: This region is further divided into three subregions:
• Eden space: Where new objects are initially allocated
• Survivor space: Where objects that survive garbage collection cycles are promoted
• From Survivor space: A temporary space used during garbage collection
• To Survivor space: A temporary space used during garbage collection

• Old generation: Wherelong-lived objects are stored

In Java 6 and earlier versions, the heap also included a permanent generation, which held metadata such as class definitions and constant pools. In Java 7, the permanent generation was merged into the heap, and in Java 8, it was replaced by the Metaspace, which is part of the non-heap memory.

Allocation and Garbage Collection

Objects are allocated in the Eden space. When the Eden space is full, a minor garbage collection (GC) cycle is triggered. During a minor GC, objects that are still reachable (still referenced by other objects) are promoted to the Survivor space. Objects that are not reachable are removed from the heap. The Survivor space is divided into two subspaces, From Survivor and To Survivor. During a minor GC, objects are copied from the Eden space to the From Survivor space. During the next minor GC, objects are copied from the From Survivor space to the To Survivor space. Objects that survive multiple minor GC cycles are promoted to the old generation. The old generation is where long-lived objects are stored. When the old generation is full, a major GC cycle is triggered. During a major GC,all objects in the old generation are marked as unreachable and removed from the heap.

Tuning Heap Memory

The size of the heap memory can be tuned to improve performance. The following parameters can be used to tune the heap memory:

• -Xms: Sets the initial size of the heap
• -Xmx: Sets the maximum size of the heap
• -XX:NewSize: Sets the initial size of the young generation
• -XX:MaxNewSize: Sets the maximum size of the young generation
• -XX:SurvivorRatio: Sets the ratio of the Survivor space to the Eden space

Monitoring Heap Memory

The heap memory can be monitored using various tools, such as:

• jconsole: A graphical tool that provides real-time monitoring of the JVM
• jvisualvm: A visual tool that provides advanced monitoring and profiling capabilities
• VisualVM: A third-party tool that provides comprehensive monitoring and profiling features

Conclusion

The Java heap memory is a critical part of the JVM and plays a central role in object allocation and garbage collection. By understanding the structure and management of the heap memory, developers can tune their applications to improve performance and avoid memory-related problems.

---

《Java性能权威指南》txt下载在线阅读全文,求百度网盘云资源

《Java性能权威指南》（奥克斯 (Scott Oaks)）电子书网盘下载免费在线阅读

链接:奥克斯-(Scott-Oaks)-Java性能权威指南

java中，生产环境服务器变慢，如何诊断处理？

生产环境应当有负责监控项目的一些系统。 例如，kibana上可以查看哪些接口的响应时间比较长。 数据库监控可以看到有没有慢查询，有的话去找运维要一下具体的慢查询语句，然后去分析这个慢查询产生的时间和都有哪些接口被调用了。 去看这些接口的响应时间哪个能对的上。 定位到对应的代码，进行优化。 这个是个人认为比较常见的一种情况。