Can anyone tell me what does this thing do? Also if anyone can give an example if would be helpful.
public class ConnectionManager{
private static ConnectionManager instance = null;
.....}
Here is the complete code:
package com.gollahalli.main;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
public class ConnectionManager
{
private static ConnectionManager instance = null;
private final String USERNAME = "root";
private final String PASSWORD = "root";
private final String H_CONN_STRING = "jdbc:hsqldb:data/explorecalifornia";
private final String M_CONN_STRING = "jdbc:mysql://localhost/explorecalifornia";
private DBType dbType = DBType.MYSQL;
private Connection conn = null;
private ConnectionManager() { }
public static ConnectionManager getInstance() {
if (instance == null) {
instance = new ConnectionManager();
}
return instance;
}
public void setDBType(DBType dbType) {
this.dbType = dbType;
}
private boolean openConnection() {
try {
switch (dbType) {
case MYSQL:
conn = DriverManager.getConnection(M_CONN_STRING, USERNAME, PASSWORD);
return true;
case HSQLDB:
conn = DriverManager.getConnection(H_CONN_STRING, USERNAME, PASSWORD);
return true;
default:
return false;
}
}
catch (SQLException e) {
System.err.println(e);
return false;
}
}
public Connection getConnection() {
if (conn == null) {
if (openConnection()) {
System.out.println("Connection opened");
return conn;
} else {
return null;
}
}
return conn;
}
public void close() {
System.out.println("Closing connection");
try {
conn.close();
conn = null;
} catch (Exception e) { }
}
}
There is the singleton design pattern.
It used to make sure that only one instance of a class can be created.
public class MySingletonClass {
private static MySingletonClass instance;
public synchronized static MySingletonClass getInstance() {
if (instance == null) {
instance = new MySingletonClass(); // "lazy" initialization
}
return instance;
}
/**
* private constructor can be called only inside of MySingleton class, but not from outside.
*/
private MySingletonClass() {
// your code here
}
}
So, to get an instance of this class in the code, a developer does not use the constructor.
Developer uses the static method getInstance().
MySingletonClass mySingleton = MySingletonClass.getInstance();
Please be careful with singletons. Many novice developers abuse use of singletons and use them as global variables. Don't do it :)
UPDATE:
I added synchronized to the getInstance() method to make it thread safe.
It simply declares a field called instance whose type is ConnectionManager and initializes it to null (which is redundant because that would be its default value anyway).
Most likely the class is a singleton class (only one instance is allowed from them) judging by the instance field declaration and by the name of the class.
It's called the Singleton pattern.
This is used when you need only one object of a class, the singleton. It will be construct only one time and then you can access it through getInstance().
Naive implementation
public class SingletonDemo {
//Holds the singleton
private static SingletonDemo instance = null;
//Overrides default constructor, not to instantiate another one.
//Only getInstance will construct
private SingletonDemo() { }
//Only this method can construct a singleton, always call this one
public static SingletonDemo getInstance() {
if (instance == null) { //No singleton yet, create one
instance = new SingletonDemo();
}
//return the singleton (created this time or not)
return instance;
}
}
Related
In one of the interview I got asked about to how to make sure single object is present of a Singleton class when used with multiple classloaders.I followed this SO Question but I couldn't understand how we should invoke the getClass(). As there is no information about it in the answer and as well as in the link provided in the answer. I am new to classloaders and things aren't that clear for me. I have tried the code and tried multiple combination but every time I am getting two instance instead of one.
package com.design_pattern;
import java.io.*;
import java.lang.reflect.Constructor;
public class SIngletonPattern {
public static void main(String[] args) throws Exception {
DatabaseConnection databaseConnection = DatabaseConnection.getDatabaseConnection();
ClassLoader classLoader = SIngletonPattern.class.getClassLoader();
System.out.println(classLoader);
//Class classs = classLoader.getClass().forName(DatabaseConnection.class.getName());
Class classs = DatabaseConnection.getClass(DatabaseConnection.class.getName());
System.out.println(classs.getName());
Constructor[] declaredConstructors = classs.getDeclaredConstructors();
DatabaseConnection db2 = null;
for (Constructor d:declaredConstructors) {
d.setAccessible(true);
db2 = (DatabaseConnection) d.newInstance();
}
if(db2 == null){
System.out.println("null");
}else{
System.out.println(databaseConnection == db2);
}
}
}
class DatabaseConnection implements Serializable {
private static volatile DatabaseConnection databaseConnection;
private DatabaseConnection() {
System.out.println("Instance created");
}
public static DatabaseConnection getDatabaseConnection() {
System.out.println("getInstance()");
if (databaseConnection == null) {
synchronized (DatabaseConnection.class) {
if (databaseConnection == null) {
databaseConnection = new DatabaseConnection();
}
}
}
return databaseConnection;
}
protected Object readResolve() {
return databaseConnection;
}
public static Class getClass(String classname)
throws ClassNotFoundException {
ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
if(classLoader == null)
classLoader = DatabaseConnection.class.getClassLoader();
return (classLoader.loadClass(classname));
}
}
Singleton is a service that require injection of authentication and configuration data. I end with class:
class SingleService {
private String conn;
private String user;
private String pass;
private SingleService() {
// Can throw exception!!
conn = Config.getProperty("conn");
user = Config.getProperty("user");
pass = Config.getProperty("pass");
// Can throw exception!!
internalService = tryConnect(conn, user, pass);
}
private static SingleService instance;
public static void init() {
instance = new SingleService();
}
public static synchronized SingleService getInstance() {
if (instance == null) init();
return instance;
}
}
Dedicated init() method used for exception handling during application startup to early detect initialization errors early because later we just call getInstance() and doesn't expect to get errors:
class App {
public static void main(String args[]) {
try {
Config.init("classpath:auth.properties");
SingleService.init();
} catch (Exception ex) {
logger.error("Can't init SingleService...");
System.exit()
}
doJob();
}
private static void doJob() {
SingleService.getInstance().doJob();
}
}
I worry about init() method and singleton class signature. Fill that class was designed badly but don't understand what's wrong.
Is it possible to move away initialization from getSingleton() and synchronized and preserving control on exception during initialization?
This is how I would code it so you can throw exceptions if needed but still have a thread safe singleton.
enum SingleService {
INSTANCE;
private String conn;
private String user;
private String pass;
private SingleService instance;
public synchronized void init(Config config) throws SomeException {
// don't leave in a half state if we fail.
internalService = null;
conn = config.getProperty("conn");
user = config.getProperty("user");
pass = config.getProperty("pass");
internalService = tryConnect(conn, user, pass);
}
public synchronized void methodForService() {
if (internalService == null) throw new IllegalSateException();
// do work.
}
}
SingleService ss1 = SingleService.getInstance();
SingleService.init();
SingleService ss2 = SingleService.getInstance();
So ss1 is a different object than ss2 which is not what Singleton is designed for. If ss1 is modified at anytime ss2 will remain unaffected.
Fist of all you souhld not expose object creation method. If you want to check something, than go with asserts or any operation that will not corrupt instance object.
public static void checkIfValid() {
assert Config.getProperty("conn");// do not corrupt instance object
assert Config.getProperty("user");
assert Config.getProperty("pass");
}
public static synchronized SingleService getInstance() {
if (instance == null){ // only here you can initiate instance object
instance = new SingleService();
}
return instance;
}
My production code for problem I have sought:
public abstract class AbstractCaller<Port> {
abstract protected Port createPort();
protected init() {
Port port = createPort();
// heavy use of introspection/reflection on **port** object.
// Results are used later by **call** method.
}
public call() {
// Reflection based on data collected by **init** method.
}
}
public class ConcreteCaller extends AbstractCaller<ConcretePort> {
private ConcreteService service = new ConcreteService();
#Override
protected ConcretePort createPort() {
return service.getPort();
}
private static class Holder {
public static ConcreteCaller INSTANCE;
static {
INSTANCE = new ConcreteCaller();
INSTANCE.init();
}
}
public static Caller getInstance() {
return Holder.INSTANCE;
}
}
Abstract class has common init method that can only operate on fully initialized concrete class. Inner static class is used for lazy instantiation and perform init invocation.
There is no way to apply init method from superclass constructor to avoid need to call init in each implementation.
How to break singleton class in java?
Singleton Pattern ensures a class has only one instance and provides a global point of access to it.
The default constructor of the class is made private, which prevents the direct instantiation of the object by other classes.
A static modifier is applied to the instance method that returns the object as it then makes this method a class level method that can be accessed without creating an object
EDIT:
public class Singleton {
private static Singleton singleInstance;
private Singleton() {
System.out.println("Singleton Constructor Running...");
}
public static Singleton getInstance() {
if (singleInstance == null) {
synchronized (Singleton.class) {
if (singleInstance == null) {
singleInstance = new Singleton();
}
}
}
return singleInstance;
}
}
To Break Singleton Using Reflection
import java.lang.reflect.Constructor;
public class BreakSingletonUsingReflection {
public static void main(String[] args) {
Singleton instanceOne = Singleton.getInstance();
Singleton instanceTwo = null;
try {
Constructor[] constructors = Singleton.class.getDeclaredConstructors();
for (Constructor constructor : constructors) {
//Below code will destroy the singleton pattern
constructor.setAccessible(true);
instanceTwo = (Singleton) constructor.newInstance();
break;
}
} catch (Exception e) {
e.printStackTrace();
}
System.out.println(instanceOne.hashCode());
System.out.println(instanceTwo.hashCode());
}
}
1.
import java.lang.reflect.Constructor;
public class ReflectionSingletonTest {
public static void main(String[] args) {
EagerInitializedSingleton instanceOne = EagerInitializedSingleton.getInstance();
EagerInitializedSingleton instanceTwo = null;
try {
Constructor[] constructors = EagerInitializedSingleton.class.getDeclaredConstructors();
for (Constructor constructor : constructors) {
//Below code will destroy the singleton pattern
constructor.setAccessible(true);
instanceTwo = (EagerInitializedSingleton) constructor.newInstance();
break;
}
} catch (Exception e) {
e.printStackTrace();
}
System.out.println(instanceOne.hashCode());
System.out.println(instanceTwo.hashCode());
}
}
OR
2.
Singleton class:
package com.singleton.securitymgr;
public class Singleton {
private static final Singleton INSTANCE = new Singleton();
private Singleton() {
System.out.println("Singleton Constructor Running...");
}
public static final Singleton getInstance() {
return INSTANCE;
}
}
Test class:
package com.singleton.securitymgr;
import java.lang.reflect.Constructor;
public class Test {
public static void main(String[] args) throws Exception {
Singleton s = Singleton.getInstance();
Class clazz = Singleton.class;
Constructor cons = clazz.getDeclaredConstructor();
cons.setAccessible(true);
Singleton s2 = (Singleton) cons.newInstance();
}
}
The singleton will only work per classLoader. To break it, use multiple classLoaders
One more way is to Serialize the Object store it in a file and Deserialize it to get a new Object.
I have an example code of Singleton class inheritance below. However, I've not forseen if there's any hidden issue might happen with this code. Can someone analyze and give me a hint?
interface ChairIF {
public int getLeg();
public void test();
}
class ChairImpl implements ChairIF {
private static final Lock lock = new ReentrantLock();
private static ChairIF instance = null;
public static ChairIF getInstance(String clazzName) {
//get class by clazzName
Class clazz = null;
try {
clazz = Class.forName(clazzName);
} catch (ClassNotFoundException ex) {
lock.lock();
try {
if (instance == null) {
instance = new ChairImpl();
}
} finally {
lock.unlock();
}
}
//init singleton instance of clazzName
if (instance == null) {
lock.lock();
try {
if (instance == null) {
instance = (ChairIF) clazz.newInstance();
} else {
if (instance.getClass() != clazz) {
instance = (ChairIF) clazz.newInstance();
}
}
} catch (Exception ex) {
instance = new ChairImpl();
} finally {
lock.unlock();
}
} else {
lock.lock();
try {
if (!instance.getClass().getName().equals(clazz.getName())) {
instance = (ChairIF) clazz.newInstance();
}
} catch (Exception ex) {
instance = new ChairImpl();
} finally {
lock.unlock();
}
}
return instance;
}
public int getLeg() {
return 4;
}
public void test() {
throw new UnsupportedOperationException();
}
}
class ThreeLegChair extends ChairImpl {
public ThreeLegChair() {}
public int getLeg() {
return 3;
}
public void test() {
int i = 0;
while(i < 10000) {
System.out.println("i: " + i++);
}
}
}
class NoLegChair extends ChairImpl {
public NoLegChair() {}
public int getLeg() {
return 0;
}
public void test() {
int j = 0;
while(j < 5000) {
System.out.println("j: " + j++);
}
}
}
public class Test {
public static void main(String[] args) {
System.out.println(ChairImpl.getInstance("ThreeLegChair").getLeg());
System.out.println(ChairImpl.getInstance("NoLegChair").getLeg());
/***
TODO: build logic to run 2 test() simultaneously.
ChairImpl.getInstance("ThreeLegChair").test();
ChairImpl.getInstance("NoLegChair").test();
****/
}
}
As you can see, I did put some test code in 2 subclasses. ThreeLegChair is to loop from 0 to 10000 and print it out. NoLegChair is to loop only from 0 to 5000 and print it out.
The result I got in the console log is correct. ThreeLegChair printed i from 0 to 10000. NoLegChair printed j from 0 to 5000.
Please share me your thought :)
Singleton pattern is achieved using the concept of private constructor i.e. the class itself is responsible for creating single instance of the class (singleton) and preventing other classes from creating objects.
Now as the constructor is private, you cannot inherit the singleton class at first place. In your case, I do not see a private constructor which makes it vulnerable to object creation from other classes accessing it.
Singleton pattern examples:
Using enumerations in Java
enum SingletonEnum {
SINGLE_INSTANCE;
public void doStuff() {
System.out.println("Singleton using Enum");
}
}
Lazy initialization approach
class SingletonClass {
private static SingletonClass singleInstance;
private SingletonClass() {
// deny access to other classes
}
// The object creation will be delayed until getInstance method is called.
public static SingletonClass getInstance() {
if (null == singleInstance) {
// Create only once
singleInstance = new SingletonClass();
}
return singleInstance;
}
}
However, the above example may not guarantee singleton behavior in multithreaded environment. It is recommended to use double checked locking mechanism to ensure that you have created a single instance of this class.
The code you post isn't an implementation of the singleton pattern.
Quite simply, you can do:
ChairImpl ci = new ChairImpl();
And instantiate as many as you want.
The traditional method of implementing the singleton pattern is the make the constructor private, have a private static field that holds the single instance of the class, and a static getInstance() method that either instantiates that instance or returns the existing one. Making that threadsafe involves either declaring it synchronized or using a locking scheme.
The private constructor bit makes it so you can't inherit from it.
That said, in Java the preferred way is using an enum which provides all the hard parts for free:
public enum MySingleton {
INSTANCE;
public int getLeg() {
return 4;
}
}
And using as:
MySingleton ms = MySingleton.INSTANCE;
int leg = ms.getLeg();
Singletons usually have private constructor. Your class is not following proper Singleton pattern. otherwise you would not be inherit your singleton class.
I would like to use db4o and I`m learning this using in youtube.com tutorial. Unfortunately I'm not able to find mistake in my code. I would like to know why I have got there error? I added all important library.
Code important class:
package data;
import com.db4o.*;
import com.db4o.config.EmbeddedConfiguration;
public class DataConnection {
private static DataConnection INSTANCE =null;
private final String PATH = "test.db4o";
private static ObjectContainer db;
private DataConnection(){}
private synchronized static void createInstance(){
if (INSTANCE ==null){
INSTANCE = new DataConnection();
INSTANCE.performConnection();
}}
public void performConnection() {
EmbeddedConfiguration config = Db4oEmbedded.newConfiguration();
db = Db4oEmbedded.openFile(config, PATH);
}
public static ObjectContainer getInstance() {
if(INSTANCE == null) createInstance();
return db;
}
public static void closeConnection() {
try{
db.close();
}
catch (Exception e) {
e.printStackTrace();
}
}
}
Here is this tutorial (important thing 5:44):
http://www.youtube.com/watch?v=dcNfkED53to
Try replacing
new DataConnection.getInstance()
with
DataConnection.getInstance()
The keyword new is only used when creating a new object. Here you are calling a static method.