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pnuz3n/MixinsExample.java

Last active Mar 6, 2020
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Simple try-out for DCI programming style (basically mixins) using Java 8, virtual default methods and dynamic proxy.
// Idea about simple DCI (data, context, interaction) using Java 8.
// Approach will force mixins to Java using virtual default methods
// and dynamic proxy. To keep it short there is no context object
// like DCI dictates.
//
// Run using
// javac MixinsExample.java; java MixinsExample
//
// Panu Wetterstrand <panu.wetterstrand@iki.fi>
import java.lang.reflect.*;
import java.lang.invoke.*;
public class MixinsExample {
public static void main(String[] args) {
// Lets have some accounts with initial balances
Account a = new AccountImpl(100);
Account b = new AccountImpl(50);
// Assign roles
TargetAccount target = assignRole(TargetAccount.class, a);
SourceAccount source = assignRole(SourceAccount.class, b);
// Print situation before
System.out.println("Source: "+source.balance());
System.out.println("Target: "+target.balance());
// Make the transfer
source.transferTo(target, 25);
// Print the results
System.out.println("Source: "+source.balance());
System.out.println("Target: "+target.balance());
}
// Account is basic data object containint the balance.
// Reason for this comes later...
static public interface Account {
public double balance();
public void deposit(double amount);
public void withdraw(double amount);
}
// Then we have the implementation
public static class AccountImpl implements Account {
double balance = 0;
public AccountImpl(double balance){
this.balance = balance;
}
public double balance(){
return balance;
}
public void deposit(double amount){
balance += amount;
}
public void withdraw(double amount){
balance -= amount;
}
}
// We have a role that acts as a source account.
// When this extends the Account interface we can use
// this.withdraw...
static public interface SourceAccount extends Account {
default public void transferTo(TargetAccount target, double amount) {
this.withdraw(amount);
target.deposit(amount);
}
}
// Just to mark the role of target account.
static public interface TargetAccount extends Account {
}
// Creates proxy that makes it possible to "cast" data objects
// to act in some role. This is where the magic happens...
static <T> T assignRole(Class<T> roleClass, final Object o){
return (T) Proxy.newProxyInstance(
roleClass.getClassLoader(),
new Class<?>[] { roleClass },
(Object proxy, Method method, Object[] args) -> {
if (method.isDefault()) {
// See https://rmannibucau.wordpress.com/2014/03/27/java-8-default-interface-methods-and-jdk-dynamic-proxies/#comment-1332
final Class<?> declaringClass = method.getDeclaringClass();
final MethodHandles.Lookup lookup = MethodHandles.publicLookup()
.in(declaringClass);
// ensure allowed mode will not check visibility
final Field f = MethodHandles.Lookup.class.getDeclaredField("allowedModes");
final int modifiers = f.getModifiers();
if (Modifier.isFinal(modifiers)) { // should be done a single time
final Field modifiersField = Field.class.getDeclaredField("modifiers");
modifiersField.setAccessible(true);
modifiersField.setInt(f, modifiers & ~Modifier.FINAL);
f.setAccessible(true);
f.set(lookup, MethodHandles.Lookup.PRIVATE);
}
return lookup
.unreflectSpecial(method, declaringClass)
.bindTo(proxy)
.invokeWithArguments(args);
}
return method.invoke(o,args);
}
);
}
}
@softwarehangar7

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@softwarehangar7 softwarehangar7 commented Mar 6, 2020

Hi! This is very interesting. Is it based on the DCI by James Coplien?
I'm starting to study this, and still a bit puzzled.

@pnuz3n

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@pnuz3n pnuz3n commented Mar 6, 2020

Hi! This is very interesting. Is it based on the DCI by James Coplien?
I'm starting to study this, and still a bit puzzled.

I think Data Context Interaction predates James Coplien, but yes this is simple Data Context Interaction implementation idea.

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