Java (1)--Basic overview

1.Java Running development environment

Sun released JDK1.0 (first edition) in early 1996. This version consists of two parts: the runtime Environment (JRE) and the development environment (JDK).

(1) The operating environment includes: Core API, Integration API, user interface API, publishing technology, Java Virtual Machine (JVM) 5 parts;

(2) The development environment includes: Compiling the Java program compiler (Javac command).

2.Java program operating mechanism and JVM:

(1) Instead of generating a platform-specific machine code, the Java language compiles a platform-independent bytecode (Xxx.class), which must be interpreted with the Java interpreter. For Java, the compilation and interpretation are indispensable, such as:

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(2) Java Virtual machine, the JVM (Java Vsan), is responsible for interpreting the execution bytecode. Java bytecode is not intended for any platform, only for the JVM. The JVM is different on different platforms, but they all provide the same interface;

Compilation and operation of 3.Java programs

(1) Using Notepad, compile the command: javac-d. Helloworld.java (-D indicates the directory in which the. class file is generated,.); Run command: Java Java class name (note: The runtime Java command is followed by the class name, not the. class file name, more than the source Java file name).

(2) Note that Java is case-sensitive.

4. About Java programs:

(1) The class is the smallest unit of a Java program. The simplest Java program is the program Class a{}, which only has an empty class, but the class can only be compiled through, but not run, because the Java program requires the main method to run the portal;

(2) The main method is almost fixed, that is, must be modified by public static void, the argument must be a string array type (string[] args), the General Java program has only one main method, Other methods are called directly or indirectly by the Main method;

5. Garbage collection mechanism

(1) The memory allocation and collection of Java programs are performed automatically by the JRE in the background, and the JRE recycles memory that is no longer used, known as garbage collection (garbage Collection-GC);

(2) The JRE will provide a background thread to detect and control, generally in the CPU idle or low memory of the automatic garbage collection, and program developers can not accurately control the time and sequence of garbage collection;

(3) The heap memory holds the instance object of all classes, if it is no longer referenced by the program, there is a garbage collection mechanism automatically released;

(4) In addition to releasing useless objects, the JVM can also be aware of memory fragmentation (fragmentation is the free memory area allocated to object memory), and the JVM allocates the compiled memory to new objects;

(5) A few notes on GC:

A.GC can only reclaim memory resources, for other physical resources: database connection, disk I/O resources are powerless;

B. To make the useless object recyclable as soon as possible, assign the object null;

C.runtime.getruntime (). GC (); and System.GC () are only recommended for recycling garbage;

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Java (1)--Basic overview