Java Reflection

General info

• To discover information about objects and their classes at runtime.
• RTTI -- Runtime type information.
• Usage.
• instanceof operator.
• API mostly used.
• Methods mostly used.
• Manipulate a class.
• Security model.

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Usage

• Construct new class instances and new arrays
• Access and modify fields of objects and classes
• Invoke methods on objects and classes
• Access and modify elements of arrays

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instanceof operator

• Determine whether the runtime type of an object is of a particular type or a subclass of that particular type.
• Cannot be used to test primitive type.

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API mostly used

• java.lang.Class
• java.lang.reflect.Field
• java.lang.reflect.Method
• java.lang.reflect.Array
• java.lang.reflect.Constructor

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Methods mostly used

• Class literals: T.class

  boolean.class Boolean.TYPE
  char.class Character.TYPE
  ...
  double.class Double.TYPE
  void.class Void.TYPE
  

• Find a class. You must know the class name:

  Class cls = Class.forName(aClassName);
  

• Find a super class.

  Class supr = cls.getSuperclass();
  Interface[] ifs = cls.getInterfaces();
  

• Find an instance of a class:

  Object obj = cls.newInstance();
  or 
  ob.getClass().newInstance();
  

• Find fields info:

  Field[] fields = cls.getFields();
  

• Find methods info:

  Method[] mths = cls.getMethods();
  

• Find modifiers info:

  int mods = cls.getModifiers();
  Modifier code:
  1 -- public
  2 -- private
  4 -- protected
  8 -- static
  16 -- final
  32 -- synchronized
  64 -- volatile
  128 -- transient
  256 -- native
  
  1024 -- abstract
  
  Check modifier:
  if (Modifier.isPublic(mods))
     ...
  
  Print a modifier:
  String s = MOdifier.toString(mods);
  

• Other methods:

  isInterface();
  isArray();
  isPrimitive();
  getConstructors();
  

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Manipulate a class

• Manipulate fields.

  Field[] flds = cls.getFields();
  //Check modifier first, if necessary.
  int mods = flds[i].getModifiers();
  flds[i].set(obj, value);
  flds[i].setBoolean(obj, b);
  flds[i].setLong(obj, l);
  ...
  

• Invoke a method.

  Method m = cls.getMethod(name, paraTypes);
  Object ob = m.invoke(obj, args);
  
  public Method getMethod(String nm, Class[] paramTypes);
  public Object invoke(Object ob, Object[] args);
  

• Manipulate an array.

  public Class getComponentType();
  public static int getLength(Object array);
  public static Object newInstance(Class type, int len);
  
  public static Object get(Object array, int idx);
  public void set(Object array, int idx, Object value);
  

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Security Model

• Security Manager controls access to the core Reflection API on a class-by-class basis.
• Standard Java programming language access control is enforced when:
  • A Field is used to get or set a field value
  • A Method is used to invoke a method
  • A Constructor is used to create and initialize a new instance of a class

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