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

Composition or inheritance

Think of containment as a has a relationship. A car "has an" engine, a person "has a" name, etc.

Think of inheritance as an is a relationship. A car "is a" vehicle, a person "is a" mammal, etc.

Useful tips

Iterate map

- for (Map.Entry<String, String> entry : map.entrySet()) { }

List to array

- int[] arr= new int[list.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](https://stackoverflow.com/questions/6270132/create-a-custom-event-in-java)

Try with resource

The try-with-resources statement is a try statement that declares one or more resources. A resource is an object that must be closed after the program is finished with it. The try-with-resources statement ensures that each resource is closed at the end of the statement. Any object that implements java.lang.AutoCloseable, which includes all objects which implement java.io.Closeable, can be used as a resource.

In short no need add manual finally block to close resources.

    try (InputStream caInput = new ByteArrayInputStream(decode)) {
      // Generate the CA Certificate from the raw resource file

      Certificate ca = CertificateFactory.getInstance("X.509").generateCertificate(caInput);

      // Load the key store using the CA
      KeyStore keyStore = KeyStore.getInstance(KeyStore.getDefaultType());
      keyStore.load(null, null);
      keyStore.setCertificateEntry("ca", ca);

      // Initialize the TrustManager with this CA
      TrustManagerFactory tmf = TrustManagerFactory.getInstance(TrustManagerFactory.getDefaultAlgorithm());
      tmf.init(keyStore);

      // Create an SSL context that uses the created trust manager
      SSLContext sslContext = SSLContext.getInstance("TLS");
      sslContext.init(null, tmf.getTrustManagers(), new SecureRandom());

      return sslContext.getSocketFactory();

    } catch (Exception ex) {
      logger.error(ex.getMessage());
      throw new RuntimeException(ex);
    }

JAVA SE

Java Collection

HashMap and HashTable

• Hashtable is synchronized, whereas HashMap is not. This makes HashMap better for non-threaded applications, as unsynchronized Objects typically perform better than synchronized ones.

• Hashtable does not allow null keys or values. HashMap allows one null key and any number of null values.

• One of HashMap's subclasses is LinkedHashMap, so in the event that you'd want predictable iteration order (which is insertion order by default), you could easily swap out the HashMap for a LinkedHashMap. This wouldn't be as easy if you were using Hashtable.

• HashMap is not thread-safe, if you do want to have thread-safe one then use Collections.synchronizedMap(),

• HashTable is thread-safe, but is treated as legacy.

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)

SSL

Java use TLSv1.2 by default since Java 8

When you saw some error like this SSL handshake exception: “Algorithm constraints check failed: MD5withRSA”, probably you used MD5 as message digest when generate a CA. You should regenerate a new CA using sha1, because the MD5 has been found to be insecure.

Some more info https://stackoverflow.com/questions/21218217/ssl-handshake-exception-algorithm-constraints-check-failed-md5withrsa

Monotonic clock

Note: You can go to the go page to check out more background knowledge

long startTime = System.nanoTime();
// ... the code being measured ...
long estimatedTime = System.nanoTime() - startTime;

Some code which uses the wall clock

long start = System.currentTimeMillis();
// Do something exceptional
long elapsed = System.currentTimeMillis() - start;