Beginner's word No4 (Java command, JVM)

field -Variable name declared in the class -A unit that resists records. (Other meaning fields / areas, etc.)

java command

A command to launch a Java application. (Run a Java program)

Command options

** Example: An example of running Test.jar. Set "Taro" as a parameter. ** ** ＞＞ java -jar Test.jar ** "Taro" **

• Other command systems will be described together.

JVM(Java Vitual Machine) Software for running Java programs. In other words, "Java virtual machine" and "Java VM"

Java is a language created with the idea that Java runs on any machine.

How the JVM works

① Java program ⇒ ② JVM ⇒ ③ OS Interpret ①, convert it to a code that can be executed by the OS of ③, and operate it. (2) JVM is indispensable to run programs written in Java without depending on the platform.

Convert text code to binary code (machine language) from ① to ② Read the binary code from ② to ③ and convert it to native code

JVM configuration

There are roughly three areas.

1. New area
2. Old area
3. Permanent area There is. Heap in areas 1 and 2. Area 3 is a non-heap area.

heap

A memory area used by a system such as an OS or application.

• The setting of the JVM heap area can be changed freely as an option.

1. 1. New area ⇒It is further classified into three areas. ⇒ Placed in the Eden area when the object is instantiated ⇒ When the memory of Eden becomes full, it is stored in the From area and To area to save the object.
2. Old area ⇒ Information that has been used for a long time is managed by the object stored in the New area. ⇒ When the Old is full, a process called Full GC frequently occurs.

• FullGC is a heavy process, so the system will stop while it is running.

3. 3. Permanent area ⇒ Information such as classes and methods is stored. [Supplement] GC stands for Garbage Collection. ⇒ Organize the used memory area in the memory area and create a free area