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How to Inject a runtime created string to another class using Spring

At the startup, I am creating a string according to the database type in class A. Then I want to inject that string value to class B after the setBitAndSqlExpression() function is called. After the injection I want to use that variable in Class B. Class B is executed if the user makes a request after run time init.
Note that they are in different packages. I cannot pass them via the constructor.
class A { //code first enters here
public String BITAND_SQL_EXPRESSION;
public void setBitAndSqlExpression() {
try {
String driverName = dataSource.getConnection().getMetaData().getDriverName();
if (driverName.contains("PostgreSQL")) {
BITAND_SQL_EXPRESSION = "%s" + "::INTEGER" + " & %s = %s";
} else {
BITAND_SQL_EXPRESSION = "BITAND (%s,%s) = %s";
}
} catch (Exception e) {
LOGGER.warn("Driver cannot be found. Setting operations will not work correctly.");
}
}
}
class B { //enters here if only user makes a request after class A creation.
public String BITAND_SQL_EXPRESSION; // want to use the value at class A
public B(
...//codes and constructors.
}

Is a factory method possible for you ?
It would be in the package of the B class :
public interface IB {
methodA();
methodB();
}
class B implements IB{ //enters here if only user makes a request after class A creation.
public String BITAND_SQL_EXPRESSION; // want to use the value at class A
public B(
...//codes and constructors.
}
public final class BFactory {
public static IB create(String sqlExpression) {
return new B(sqlExpression);
}
}
Then in the package of class A you could call this factory
BFactory.create(BITAND_SQL_EXPRESSION);
A would depend of an interface and not the B implementation.

Unity-Encrypting String using the Decorator Pattern

I'm trying to apply the decorator pattern to making an object that encrypts a word into a certain encryption, like the L337 method, which replaces letters like 9 with g, or 4 with r. Basically, I want to type a word into an inputfield and show the encrypted word in a text object. But I can't get the L337 decorator to inherit from the main decorator class. It won't accept the keyword 'super', so I tried the base word, but then when I implement Encrypt, it won't take the object newEncryption. Could someone help me figure out how to put this pattern together please?
I basically know what the decorator pattern is. It's making an object, making a basic decorator, and making a specific decorator, and instantiating the object with the decorating for exclusive methods and features.
public class Encryption : MonoBehaviour
{
public static InputField inputBox;
public static Text outputText;
public interface IEncryption { void Encrypt(); }
public class TextEncryption : IEncryption
{
public void Encrypt()
{
string currentText = inputBox.text;
outputText.text = currentText;
}
}
public abstract class encryptionDecorator : IEncryption
{
protected IEncryption tempEncryption;
public encryptionDecorator(IEncryption newEncryption)
{
tempEncryption = newEncryption;
}
public void Encrypt()
{
tempEncryption.Encrypt();
}
}
public class L337EncryptionDecorator : encryptionDecorator
{
public L337EncryptionDecorator(IEncryption newEncryption) : base(newEncryption)
{
print("Encrypting L337 Code");
}
public void Encrypt()
{
}
}
}

I think you actually want to use tempEncryption, but you didnt really tell where you could not use newEncryption so im guessing.
But anyway, I hope this will clear some things up. Its slightly edited from your code so I didnt need to put GUI stuff, but you could just CnP it to unity.
using UnityEngine;
public class Encryption : MonoBehaviour {
public interface IEncryption {
void Encrypt();
}
public class TextEncryption : IEncryption {
public void Encrypt() {
}
}
public abstract class EncryptionDecorator : IEncryption {
protected IEncryption tempEncryption;
public EncryptionDecorator(IEncryption newEncryption) {
//this will be called when you override the constructor
Debug.Log("In EncryptionDecorator constructor: " + newEncryption.GetType());
tempEncryption = newEncryption;
}
//if you are going to override a method in a child class,
//declare it either abstract ("no body; passes implementation to child") or
//virtual ("allows for a base implementation")
public virtual void Encrypt() {
Debug.Log("In EncryptionDecorator.Encrypt(): " + tempEncryption.GetType());
tempEncryption.Encrypt();
}
}
public class L337EncryptionDecorator : EncryptionDecorator {
public L337EncryptionDecorator(IEncryption newEncryption) : base(newEncryption) {
//newEncryption is a parameter, think of it as sort of a local variable.
//but since you pass it down to the parent class, it gets assigned to tempEncryption
//the base-class constructor is called first!
Debug.Log("In L337EncryptionDecorator constructor: " + newEncryption.GetType());
}
//this overrides the base implementation. you can call it with
//base.Encrypt() though.
public override void Encrypt() {
//you have no parameters here, but you could use the inherited variable tempEncryption because you declared it protected
Debug.Log("In L337EncrytionDecorator.Encrypt(): " + tempEncryption.GetType());
//base refers to the base class
base.Encrypt();
}
}
void Start() {
IEncryption encryption = new L337EncryptionDecorator(new TextEncryption());
encryption.Encrypt();
}
}
or maybe i missed what this is all about?!

Java: Is it possible to always execute a certain function before other functions are called? (Like #Before in JUnit)

Is there a way to always execute a function before any other function of a class is called?
I have a class where I need to refresh some fields always before any function is called:
public class Example {
private int data;
public void function1(){
}
public void function2(){
}
//#BeforeOtherFunction
private void refresh(){
// refresh data
}
}
Because it seems to be bad programming, I don't want to call refresh at the beginning of every other function. Since other persons are going to work on this project as well, there would be the danger, that somebody extends the calls and doesn't call refresh.
JUnit has a solution for this with the #Before-Annotation. Is there a way to do this in other classes as well?
And by the way: If you know a programming pattern wich solves this problem in another way than executing a function everytime any function is called, that would be very helpful, too!

Use a dynamic proxy in which you can filter to those methods before which your specific "before" method should be called. And call it in those cases before dispatching the call. Please see the answer from How do I intercept a method invocation with standard java features (no AspectJ etc)?
UPDATE:
An interface is needed to be separated for the proxy. The refresh() method cannot remain private. It must be public and part of the interface (which is not nice here) to be able to be called from the proxy.
package CallBefore;
public interface ExampleInterface {
void function1();
void function2();
void otherFunction();
void refresh();
}
Your class implements that interface:
package CallBefore;
public class Example implements ExampleInterface {
#Override
public void function1() {
System.out.println("function1() has been called");
}
#Override
public void function2() {
System.out.println("function2() has been called");
}
#Override
public void otherFunction() {
System.out.println("otherFunction() has been called");
}
#Override
public void refresh() {
System.out.println("refresh() has been called");
}
}
The proxy which does the trick. It filters the needed methods and calls refresh().
package CallBefore;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
public class ExampleProxy implements InvocationHandler {
private ExampleInterface obj;
public static ExampleInterface newInstance(ExampleInterface obj) {
return (ExampleInterface) java.lang.reflect.Proxy.newProxyInstance(obj.getClass().getClassLoader(),
obj.getClass().getInterfaces(), new ExampleProxy(obj));
}
private ExampleProxy(ExampleInterface obj) {
this.obj = obj;
}
#Override
public Object invoke(Object proxy, Method m, Object[] args) throws Throwable {
Object result;
try {
if (m.getName().startsWith("function")) {
obj.refresh();
}
result = m.invoke(obj, args);
} catch (InvocationTargetException e) {
throw e.getTargetException();
} catch (Exception e) {
throw new RuntimeException("unexpected invocation exception: " + e.getMessage());
}
return result;
}
}
The usage:
package CallBefore;
public class Main {
public static void main(String[] args) {
ExampleInterface proxy = ExampleProxy.newInstance(new Example());
proxy.function1();
proxy.function2();
proxy.otherFunction();
proxy.refresh();
}
}
Output:
refresh() has been called
function1() has been called
refresh() has been called
function2() has been called
otherFunction() has been called
refresh() has been called

This may not solve your exact problem but at least could be a starting point if you are allowed considering a re-design. Below is a simple implementation but with some small touches I believe you can achieve a more elegant solution. BTW, this is called Dynamic Proxy Pattern.
First thing you need is an interface for your class.
public interface Interface {
void hello(String name);
void bye(String name);
}
public class Implementation implements Interface {
#Override
public void hello(String name) {
System.out.println("Hello " + name);
}
#Override
public void bye(String name) {
System.out.println("Bye " + name);
}
}
Then java.lang.reflect.Proxy class comes to help. This class is able to create an instance for a given interface at runtime. It also accepts an InvocationHandler which helps you to capture method calls and looks like this.
public class InvocationHandlerImpl implements InvocationHandler {
private final Object instance;
public InvocationHandlerImpl(Object instance) {
this.instance = instance;
}
#Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
Object result;
try {
System.out.println("Before");
result = method.invoke(instance, args);
System.out.println("After");
} catch (Exception e){
e.printStackTrace();
throw e;
} finally {
System.out.println("finally");
}
return result;
}
}
After all your client code will look like this.
Interface instance = new Implementation();
Interface proxy = (Interface)Proxy.newProxyInstance(
Interface.class.getClassLoader(),
new Class[] { Interface.class },
new InvocationHandlerImpl(instance));
proxy.hello("Mehmet");
proxy.bye("Mehmet");
Output for this code is
Before
Hello Mehmet
After
finally
Before
Bye Mehmet
After
finally

I would define getters for every field and do the refreshment inside the getter. If you want to avoid unrefreshed access to your private fields at all, put them in a superclass (together with the getters which call refresh).
Depending on your project structure, it may be also sensible to introduce a separate class for all data that is regularly refreshed. It can offer getters and avoid that anyone accesses the non-refreshed fields.

Not in Java SE, but if you are using Java EE, you could use interceptors.
For standalone applications, you could consider using a bytecode manipulation framework, like javassist.

You can have a protected getter method for data. Access getData method instead of using data field. Child classes will see only getData and will have updated data every time.
public class Example {
private int data;
public void function1(){
}
public void function2(){
}
protected int getData(){
refresh();
return data;
}
//#BeforeOtherFunction
private void refresh(){
// refresh data
}
}

It is better to write another method which will be made protected(accessible to the child classes) which will call first the refresh method and then call the function.
This way the data would be refreshed before the function is called everytime(As per your requirement).
eg:
protected void callFunction1(){
refresh();
function();
}
Thanks,
Rajesh

You should use Decorator in this case. Decorator is a good choice for something like interceptor. Example here: https://msdn.microsoft.com/en-us/library/dn178467(v=pandp.30).aspx

dynamic object creation in java

I have the below code.
public class Test {
public static void main(String args[])
{
int i = 0;
if(i==0){
Beer obj = new Beer();
}
else {
Rum obj = new Rum();
}
System.out.println(obj.brand); //doesn't work
} }
class Drink {
}
class Beer extends Drink{
public String brand = "BeerBrand"; }
class Rum extends Drink{
public String brand = "RumBrand"; }
Is there an way to make the above work without using function overriding or dynamic class loading?
All classes are dynamically loaded in JVM there is no static loading like in C. Is this correct?

Drink should be an abstract class and provide an abstract member getBrand() or similar, overridden by Beer and Rum.
Then you'd do something like:
Drink d = null;
if (...) {
d = new Beer();
}
so you instantiate the reference appropriately. Because it's still of type Drink you can reference the brand. The Drink reference will let you access anything drink-able, and the implementation provides the specifics. Note that Drink is abstract, since you can't instantiate a Drink - you have to be more specific.
To answer your further questions, you could provide a base method and do something like:
if (this instanceof Beer) {
...
}
to avoid overriding. But why would you ?
To answer your second question, classes are dynamically loaded by the JVM upon reference. You can watch that occur by setting the -verbose flag on the JVM.

This code won't work because scope of 'obj' is only within if-else block.
You need to declare it above if-else block of type Drink.

Is there an way to make the above work without using function overriding or dynamic class loading?
The only alternative is to use reflections, but fixing the design of the classes would be much simpler/better
All classes are dynamically loaded in JVM there is no static loading like in C. Is this correct?
Yes. They can be dynamically loaded more than once and even unloaded.
Using an object orientated approach would look like this.
public class Test {
public static void main(String... args) {
Drink drink;
if (args.length == 0) {
drink = new Beer();
} else {
drink = new Rum();
}
System.out.println(drink.getBrand());
}
}
interface Drink {
public String getBrand();
}
class Beer implements Drink {
#Override
public String getBrand() {
return "BeerBrand";
}
}
class Rum implements Drink {
#Override
public String getBrand() {
return "RumBrand";
}
}

General OO question about inheritance and extensibility

So, in a single parent inheritance model what's the best solution for making code extensible for future changes while keeping the same interface (I'd like to emphasize the fact that these changes cannot be known at the time of the original implementation, the main focus of my question is to explore the best mechanism/pattern for supporting these changes as they come up)? I know that this is a very basic OO question and below I provide example of how I've been going about it, but I was wondering if there a better solution to this common problem.
Here's what I've been doing (the example code is in Java):
In the beginning, the following two classes and interface are created:
public class Foo
{
protected int z;
}
public interface FooHandler
{
void handleFoo(Foo foo);
}
public class DefaultFooHandler implements FooHandler
{
#Override
public void handleFoo(Foo foo)
{
//do something here
}
}
The system uses variables/fields of type FooHandler only and that object (in this case DefaultFooHandler) is created in a few, well-defined places (perhaps there's a FooHandlerFactory) so as to compensate for any changes that might happen in the future.
Then, at some point in the future a need to extend Foo arises to add some functionality. So, two new classes are created:
public class ImprovedFoo extends Foo
{
protected double k;
}
public class ImprovedFooHandler extends DefaultFooHandler
{
#Override
public void handleFoo(Foo foo)
{
if(foo instanceof ImprovedFoo)
{
handleImprovedFoo((ImprovedFoo)foo);
return;
}
if(foo instanceof Foo)
{
super.handleFoo(foo);
return;
}
}
public void handleImprovedFoo(ImprovedFoo foo)
{
//do something involving ImprovedFoo
}
}
The thing that makes me cringe in the example above is the if-statements that appear in ImprovedFooHandler.handleFoo
Is there a way to avoid using the if-statements and the instanceof operator?

First of all the code you wrote won't work.
Each time you see instanceof and if...else together be very careful. The order of these checks is very important. In your case you'll never execute handleImpovedFoo. Guess why :)
It's absolutely normal you have these instanceof statements. Sometimes it's the only way to provide different behavior for a subtype.
But here you can use another trick: use simple Map. Map classes of foo-hierarchy to instances of fooHandler-hierarchy.
Map<Class<? extends Foo>, FooHandler> map ...
map.put( Foo.class, new FooHandler() );
map.put( ImprovedFoo.class, new ImprovedFooHandler() );
Foo foo ...; // here comes an unknown foo
map.get( foo.getClass() ).handleFoo( foo );

The best way of handling this depends too much on the individual case to provide a general solution. So I'm going to provide a number of examples and how I would solve them.
Case 1: Virtual File System
Clients of your code implement virtual file systems which enable them to operate any sort of resource which can be made to look like a file. They do so by implementing the following interface.
interface IFolder
{
IFolder subFolder(String Name);
void delete(String filename);
void removeFolder(); // must be empty
IFile openFile(String Name);
List<String> getFiles();
}
In the next version of your software you want to add the ability to remove a directory and all it contents. Call it removeTree. You cannot simply add removeTree to IFolder because that will break all users of IFolder. Instead:
interface IFolder2 implements IFolder
{
void removeTree();
}
Whenever a client registers an IFolder (rather then IFolder2), register
new IFolder2Adapter(folder)
Instead, and use IFolder2 throughout your application. Most of your code should not be concerned with the difference about what old versions of IFolder supported.
Case 2: Better Strings
You have a string class which supports various functionality.
class String
{
String substring(int start, end);
}
You decide to add string searching, in a new version and thus implement:
class SearchableString extends String
{
int find(String);
}
That's just silly, SearchableString should be merged into String.
Case 3: Shapes
You have a shape simulation, which lets you get the areas of shapes.
class Shape
{
double Area();
static List<Shape> allShapes; // forgive evil staticness
}
Now you introduce a new kind of Shape:
class DrawableShape extends Shape
{
void Draw(Painter paint);
}
We could add a default empty Draw method to Shape. But it seems incorrect to have Shape have a Draw method because shapes in general aren't intended to be drawn. The drawing really needs a list of DrawableShapes not the list of Shapes that is provided. In fact, it may be that DrawableShape shouldn't be a Shape at all.
Case 4: Parts
Suppose that we have a Car:
class Car
{
Motor getMotor();
Wheels getWheels();
}
void maintain(Car car)
{
car.getMotor().changeOil();
car.getWheels().rotate();
}
Of course, you know somewhere down the road, somebody will make a better car.
class BetterCar extends Car
{
Highbeams getHighBeams();
}
Here we can make use of the visitor pattern.
void maintain(Car car)
{
car.visit( new Maintainer() );
}
The car passes all of its component parts to calls into ICarVisitor interface allowing the Maintainer class to maintain each component.
Case 5: Game Objects
We have a game with a variety of objects which can be seen on screen
class GameObject
{
void Draw(Painter painter);
void Destroy();
void Move(Point point);
}
Some of our game objects need the ability to perform logic on a regular interval, so we create:
class LogicGameObject extends GameObject
{
void Logic();
}
How do we call Logic() on all of the LogicGameObjects? In this case, adding an empty Logic() method to GameObject seems like the best option. Its perfectly within the job description of a GameObject to expect it to be able to know what to do for a Logic update even if its nothing.
Conclusion
The best way of handling this situations depends on the individual situation. That's why I posed the question of why you didn't want to add the functionality to Foo. The best way of extending Foo depends on what exactly you are doing. What are you seeing with the instanceof/if showing up is a symptom that you haven't extended the object in the best way.

In situations like this I usually use a factory to get the appropriate FooHandler for the type of Foo that I have. In this case there would still be a set of ifs but they would be in the factory not the implementation of the handler.

Yes, don't violate LSP which is what you appear to be doing here. Have you considered the Strategy pattern?

This looks like a plain simple case for basic polymorphism.Give Foo a method named something like DontWorryI'llHandleThisMyself() (um, except without the apostrophe, and a more sensible name). The FooHandler just calls this method of whatever Foo it's given. Derived classes of Foo override this method as they please. The example in the question seems to have things inside-out.

With the visitor pattern you could do something like this,
abstract class absFoo {}
class Foo extends absFoo
{
protected int z;
}
class ImprovedFoo extends absFoo
{
protected double k;
}
interface FooHandler {
void accept(IFooVisitor visitor, absFoo foo);
}
class DefaultFooHandler implements FooHandler
{
public void accept(IFooVisitor visitor, absFoo foo)
{
visitor.visit(this, foo);
}
public void handleFoo(absFoo foo) {
System.out.println("DefaultFooHandler");
}
}
class ImprovedFooHandler implements FooHandler
{
public void handleFoo(absFoo foo)
{
System.out.println("ImprovedFooHandler");
}
public void accept(IFooVisitor visitor, absFoo foo) {
visitor.visit(this, foo);
}
}
interface IFooVisitor {
public void visit(DefaultFooHandler fooHandler, absFoo foo);
public void visit(ImprovedFooHandler fooHandler, absFoo foo);
}
class FooVisitor implements IFooVisitor{
public void visit(DefaultFooHandler fHandler, absFoo foo) {
fHandler.handleFoo(foo);
}
public void visit(ImprovedFooHandler iFhandler, absFoo foo) {
iFhandler.handleFoo(foo);
}
}
public class Visitor {
public static void main(String args[]) {
absFoo df = new Foo();
absFoo idf = new ImprovedFoo();
FooHandler handler = new ImprovedFooHandler();
IFooVisitor visitor = new FooVisitor();
handler.accept(visitor, idf);
}
}
But this does not guarantee only Foo can be passed to DefaultFooHandler. It allows ImprovedFoo also can be passed to DefaultFooHandler. To overcome, something similar can be done
class Foo
{
protected int z;
}
class ImprovedFoo
{
protected double k;
}
interface FooHandler {
void accept(IFooVisitor visitor);
}
class DefaultFooHandler implements FooHandler
{
private Foo iFoo;
public DefaultFooHandler(Foo foo) {
this.iFoo = foo;
}
public void accept(IFooVisitor visitor)
{
visitor.visit(this);
}
public void handleFoo() {
System.out.println("DefaultFooHandler");
}
}
class ImprovedFooHandler implements FooHandler
{
private ImprovedFoo iFoo;
public ImprovedFooHandler(ImprovedFoo iFoo) {
this.iFoo = iFoo;
}
public void handleFoo()
{
System.out.println("ImprovedFooHandler");
}
public void accept(IFooVisitor visitor) {
visitor.visit(this);
}
}
interface IFooVisitor {
public void visit(DefaultFooHandler fooHandler);
public void visit(ImprovedFooHandler fooHandler);
}
class FooVisitor implements IFooVisitor{
public void visit(DefaultFooHandler fHandler) {
fHandler.handleFoo();
}
public void visit(ImprovedFooHandler iFhandler) {
iFhandler.handleFoo();
}
}
public class Visitor {
public static void main(String args[]) {
FooHandler handler = new DefaultFooHandler(new Foo());
FooHandler handler2 = new ImprovedFooHandler(new ImprovedFoo());
IFooVisitor visitor = new FooVisitor();
handler.accept(visitor);
handler2.accept(visitor);
}
}