JVM Part 1: memory and garbage collection part 3 - runtime data area - overview and threads

JVM Part 1 ： Memory and garbage collection Chapter 3 – Run time data area - Overview and thread

1. Preface & summary

This section focuses on the runtime data area , That's the part below , It's in the stage after class loading is complete

When we pass the front ： Class loading -> verification -> Get ready -> analysis -> initialization After these stages are completed , We will use the execution engine to use our classes , At the same time, the execution engine will use our runtime data area

That is, the chef cooks , We put the stuff behind the chef （ Cut vegetables , The knife , seasoning ）, Compare it to the runtime data area . And cooks can be compared to executive engines , Will be prepared through the things made into exquisite dishes

Memory Is a very important system resource ,** It's hard disk and CPU Middle warehouse and Bridge , It carries the real-time operation of the operating system and application program JVM The memory layout defines Java Memory request during run 、 Distribute 、 Managed policies , To ensure the JVM Efficient and stable operation of .** Different JVM There are some differences between memory partition and management mechanism . combination JVM Virtual machine specification , Let's talk about the classic JVM Memory layout .

We use disks or networks IO Data obtained , All need to be loaded into memory first , then CPU Get data from memory for reading , In other words, memory acts as CPU The bridge between disk and disk

A complete diagram of the runtime data area

Java Virtual machine defines several kinds of runtime data areas that will be used during program running , Some of them will be created as the virtual machine starts , Destroy as virtual machine exits . Others correspond to threads one by one , These data regions corresponding to the thread are created and destroyed as the thread starts and ends .

The gray ones are private to individual threads , The red ones are shared by multiple threads . namely ：

• Every thread ： Independence includes Program counter 、 Stack 、 Local stack .
• Sharing between threads ： Pile up 、 Out of heap memory （ Permanent generation or meta space 、 Code cache ）

Description of sharing between threads

2.JVM Thread description

A thread is a running unit in a program .JVM Allows an application to have multiple threads executing in parallel .
stay Hotspot JVM in , Each thread is mapped directly to the local thread of the operating system .

• When one Java When the thread is ready to execute , At this time, a local thread of the operating system is also created .Java After thread execution is terminated , Local threads also recycle .

The operating system is responsible for scheduling all threads to any available CPU On . Once the local thread is initialized successfully , It's going to call Java In thread run（） Method .

2.1JVM System threads

If you use jconsole Or any debugging tool , You can see that there are many threads running in the background . These background threads do not include calls public static void main（String[]） Of main Threads and all this main A thread created by a thread itself .|
These main background system threads are in Hotspot JVM There are mainly the following ：

• Virtual machine threads ： The operation of this thread is need JVM The safety point will be reached . The reason these operations have to happen in different threads is that they all need JVM Reach safety point , So the pile doesn't change . The execution types of this thread include "stop-the-world" Garbage collection , Thread stack collection , Thread suspension and biased lock revocation .
• Periodic task thread ： This thread is The embodiment of time periodic events （ Like interruptions ）, They are generally used for scheduling execution of periodic operations .
• GC Threads ： This thread Yes JVM There are different kinds of garbage collection behaviors that support .
• Compile thread ： This thread will Compile bytecode into local code .
• Signal scheduling thread ： This thread receives Signal and send it to JVM, Inside it, it's handled by calling the appropriate method .