'Back-End/Java_1'에 해당되는 글 48건

JDK 환경변수 설정(configure) - Window10

1. JDK 다운

- 해당 주소에서 원하는 JDK 다운받는다

- http://www.oracle.com/technetwork/java/javase/downloads/index-jsp-138363.html

2. 환경변수 설정

1) 제어판 -> 시스템 및 보안 -> 시스템 -> 고급 시스템 설정

2) 환경변수

- 새로 만들기를 클릭한다

- JAVA_HOME 을 만들고 JDK의 위치를 설정한다

- Path 추가

- %JAVA_HOME%\bin을 추가한다

3. 확인

- cmd

- javac -version, java -version 을 입력하여 아래와 같이 나오면 된다

Garbage Collection

- Garbage Collection is process of reclaiming the runtime unused memory automatically

- It is a way to destroy the unused objects

- Java is performed automatically, Java provides better memory management

1. Advantage

- It makes java memory efficient

- It is automatically done by the garbage collector(a part of JVM)

2. How can an object be unreferenced?

- By nulling the reference

- By assigning a reference to another

- By annoymous object etc

1) By nulling a reference

Employee e = new Employee(); e = null;

2) By assigning a reference to another

Employee e1 = new Employee(); Employee e2 = new Employee(); //now the first object referred by e1 is available for garbage collection e1 = e2;  

3) By annonymous object

new Employee();  

3. finalize() method

- The finalize() is invoked each time before the object is garbage collected

- It can be used to perform cleanup processing

protected void finalize(){}  

1) point

- The garbage collector collects only those objects that are created by new keyword

- If you have created any object without new, you can use finalize method to perform cleanup processing

4. gc() method

- gc() is used to invoke the garbage collector to perform cleanup processing

- gc() is found in System and Runtime classes

public static void gc(){}  

1) point

- Garbage collection is performed by a daemon thread called GC

- This thread calls the finalize() before object is garbage collected

Shutdown Hook

- Shutdown hook can be used to perform cleanup resource or save the state when JVM shuts down normally or abruptly

- clean resource means closing log file, sending some alerts...]

- If you want to execute some code before JVM shuts down, use shutdown hook

1. When does the JVM shut down?

- user presses ctrl+c on the command prompt

- System.exit(int) method is invoked

- user logoff

- user shutdown etc..

2. The addShutdownHook(Thread hook) mehtod

- addShutdownHook() method of Runtime class is used to register the thread with the Virtual Machine

public void addShutdownHook(Thread hook){}

3. Simple Example

class MyThread extends Thread{ public void run(){ System.out.println("Shut down task completed"); } } public class Shutdown { public static void main(String[]args){ Runtime r = Runtime.getRuntime(); r.addShutdownHook(new MyThread()); System.out.println("Now main sleeping...press ctrl+c"); try{ Thread.sleep(3000); }catch(Exception e){ } } } 

Thread pool

- Thread pool represents a group of worker threads that are waiting for the job and reuse many times

- a group of fixed size threads are created

- A thread from thread pool is pulled out and assigned a job by service provider

- After completion of the job, thread is contained in the thread pool again

1. Advantage

- Better performance

- It saves time because there is no need to create new thread

2. Real time usage

- It is used in Servlet and JSP  where container creates a thread pool to process the request

3. Example

import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; class WorkerThread implements Runnable{ private String message; public WorkerThread(String s){ this.message = s; } public void run(){ System.out.println(Thread.currentThread().getName()+"Start message:"+message); //call processmessage method processmessage(); System.out.println(Thread.currentThread().getName()+"End"); } private void processmessage(){ try{ Thread.sleep(2000); }catch(InterruptedException e){ e.printStackTrace(); } } } public class ThreadPool { public static void main(String[]args){ //creating a pool of 5 thread ExecutorService executor = Executors.newFixedThreadPool(5); for(int i = 0; i<10; i++){ Runnable worker = new WorkerThread(""+i); executor.execute(worker); } executor.shutdown(); while(!executor.isTerminated()){ } System.out.println("Finish all Threads"); } }