knowledge/categories/java.md

Java GC

• GC types
  • Serial GC
  • Parallel GC
  • CMS GC
  • G1 GC
• Object type
  • Young generation minor GC
  • Old generation full GC
• System.gc()

JVM

• Method Area
  • Class name, Constant, static Constant, Class object
• Heap

  • Object instance, array

  • young generation

  • Old generation

• VM stack
  • local variable
  • dynamic links
  • operation stacks
• Native method stack
• program counter

JVM insight

Spring

• Spring boot
  • Rest controller
  • Jpa template
  • Spring scheduler
  • Annotation bean
• Spring cloud
  • Consol
  • Zuul
  • Cloud config
  • Spring cloud zookeeper( discovery and configuration)
• Spring ecurity

ORM

• Hibernate
  • validation
• JPA
  • Sprint jpa tempalte
• Mybatis

Build

• Gradle
  • groovy task
• Maven

  • XML plugins

  • Maven mirror

  • https://maven-central.storage.googleapis.com

  • http://maven.aliyun.com/nexus/content/groups/public/

• Ant
  • task based

Useful tips

• Iterate map
  • for (Map.Entry<String, String> entry : map.entrySet()) { }
• List to array
  • int[] arr= new intlist.length; list.toArray(arr);
• Object
  • immutable
  • object that can’t be modified after it’s created
    • Don’t allow subclass override methods, class to be final
    • Don’t provide setter method
    • Make all field private and final
  • mutable
• Default Timezone
  • TimeZone.setDefault(TimeZone.getTimeZone("GMT+8"))
• generics

  • Why we need?

  • e.g we have sort method, and different type(n) of objects. then you have to write n kind of search methods.

• event listener
  • https://stackoverflow.com/questions/6270132/create-a-custom-event-in-java

JAVA SE

modifier

* public
* no modifier
* protected
* priivate

Java modifier diagram

NIO

- Channels
- A channel is a kind of stream, from the channel data can be read into a buffer.
- channel implementations
	- FileChannel
		- reads data from and to files
	- DatagramChannel
		- read and write data over the network via UDP
	- SocketChannel
		- read and write data over the network visa TCP
	- ServerSocketChannel
		- allows you to listen for incoming TCP connections, like a web server. For each incoming connection a SocketChannel is created.
- Buffer

- code sample

        RandomAccessFile aFile = new RandomAccessFile("data/nio-data.txt", "rw");
            FileChannel inChannel = aFile.getChannel();

            //create buffer with capacity of 48 bytes
            ByteBuffer buf = ByteBuffer.allocate(48);

            **int bytesRead = inChannel.read(buf**); //read into buffer.
            while (bytesRead != -1) {

            **buf.flip**();  //make buffer ready for read

            while(buf.hasRemaining()){
                System.out.print((char) **buf.get**()); // read 1 byte at a time
            }

            **buf.clear**(); //make buffer ready for writing
            bytesRead = inChannel.read(buf);
            }
            aFile.close();

- properties
	- capacity
	- position
	- limit

- Steps for buffer to read and write data.
	- write data into buffer
	- call buffer.flip()
	- read data out of the buffer
	- call buffer.clear() or buffer.compact()
- Non-blocking IO
- Selectors

- A selector allows a single thread to handle multiple Channels
- [http://tutorials.jenkov.com/java-nio/index.html](http://tutorials.jenkov.com/java-nio/index.html)