Reflection Reflexion in British English

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Reflection(Reflexion in British English )

• Mo Alkhafaji

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ReflectionA powerful Java API that allows
for dynamic introspection of classes, interfaces,
methods, and fields. No similar built-in
mechanism in Pascal, C, or C.What can you do
with Reflection?It gives you the ability to
Discover interfaces, classes, methods and
fields. Examine and call classes, methods and
constructors. Discover types of objects.
Write very generic code.
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The API

• java.lang.Class This class is used to represent
  classes.
• java.lang.reflect.Constructor A special class
  for inspecting any of a class's constructor
  methods.
• java.lang.reflect.Method This class provides an
  abstract means of invoking a Java method from a
  given class or object.

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The API Cont.

• java.lang.reflect.Modifier The Modifier class
  is used to determine whether special access
  modifiers (such as private, public, or protected)
  have been applied to a method or attribute.
• java.lang.reflect.Array This class is used as
  an abstract representation of array objects.
• java.lang.reflect.Field This class is used to
  represent an attribute field.

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Practical Examples JavaBeans.
Debuggers. Class browsers.
Introspection. Too generic tasks that cannot
be grouped into interfaces or abstract classes.
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Practical Examples - Cont.

• A proxy to remote API that can change.
• Changing code without recompiling it!
• Specific requirements.
• Artificial Intelligence (AI) in learning and
  dynamic generation of logic.

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Disadvantages

• Complex. You cannot use by only looking at the
  API.
• Error prone.
• Slow because classes, methods, and fields need to
  be loaded at runtime for introspection.
• Security.
• It does not update byte code.

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Javassist

• Unlike Reflection, Javassist API lets you
  manipulate and change byte code generated by Java
  compilers.
• CtClass, CtMethod, and CtField.
• Classes do not have to be loaded into JVM!
• You can load using toClass() method in CtClass to
  load into JVM.
• You can generate new classes and methods during
  runtime.

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Javassist Cont.

• 1. ClassPool pool ClassPool.getDefault()
• 2. CtClass pt pool.get(mypackage.PNLParser")
• 3. pt.setSuperclass(pool.get(ReportParser"))
• 4. CtMethod m CtNewMethod.make("public void
  printType() System.out.println(PNL) ", pt)
• 5. pt.addMethod(m)
• 6. pt.writeFile()