2. and there we go..!
●
●
Classes like Class, Method, etc..
A running program examining itself and it's environment..
3. Some reflective jargon & The definition
●
Metadata, MetaObjects, Introspection, etc..
●
Definition
○
○
○
The ability of a running program to examine itself and its software environment introspect, and behave accordingly.
This self-examination requires a self-representation - metadata.
In an OO world, metadata is organized into objects - metaobjects.
4. Feature set
Methods of Class for field introspection:
Field getField( String name)
Returns a Field object that represents the specified public member field of this class/interface.
Field[] getFields()
Returns an array of Field objects that represents all the accessible public fields of the this
class/interface.
Field[] getDeclaredField( String name )
Returns a Field object representing the specified declared field by this class/interface.
Field[] getDeclaredFields()
Returns an array of Field objects that represents all the that represents each field declared by this
class/interface.
5. Feature set cont.d..
Methods of Class for field introspection:
Class getType(), Class getDeclaringClass(), String getName(), int
getModifiers(), Object get( Object obj ), void set( Object obj, Object
value ),
On the similar lines, there are set of methods available for Method, Constructor & Annotations.
Hierarchy of members within reflection API:
Member
Method
Field
Constructor
7. Loading & Constructing at will..(1 of 2)
●
●
What web servers do?
○ loading and executing new code..
Load at will
○
Class.forname(String className)
■
Doesn't require class name at compile time
●
Examples:
○
○
Class cls = Class.forName("java.lang.String")
Class cls = Class.forName("java.lang.String[]");
■
System.out.println(String[].class.getName());
- Prints [Ljava.lang.String;
○
○
○
Class cls = Class.forName("[Ljava.lang.String;");
Will primitives work..?
Will primitive arrays work..?
8. Loading & Constructing at will..(2 of 2)
●
Reflective Construction
○
Class.newInstance()
■
○
Doesn't require class name at compile time
java.lang.reflect.Constructor
■
cls.getConstructor( new Class[] {String.class, String.class} )
●
introspects for a constructor of cls Class that takes two String parameters.
Flavours:
getConstructor(Class[]), getDeclaredConstructor(Class[]),
getConstructors(), getDeclaredConstructors()
○
Constructing Arrays
■
■
■
■
Array.newInstance()
Array.newInstance(String.class, 5);
Array.newInstance(String[].class, 5);
Array.newInstance(String.class, new int[] {2, 3});
9. The Joy of Breaking the laws..
#1
Accessing nonpublic members
Output:
Executing private method
120
10. The Joy of Breaking the laws..
#2
Breaking Immutability:
Output:
rue
String is mutable
String is mutable
11. Back to reality..
Plugging in the security module - SecurityManager
- An object that defines a security policy for an application.
- Any action not allowed in the policy throws SecurityException .
- Application can query its security manager for allowed actions
- Default policy file location : java.homelibsecurityjava.policy
- Specifying additional policy file at run time:
java -Djava.security.manager -Djava.security.policy=someURL SomeApp
12. Mastering the cross cutting issues..
What are cross cutting issues in an application?
Business flow Vs Logging
Business flow Vs Exception Handling
Business flow Vs Atomicity
Aspect Oriented Programming
Increasing modularity by allowing separation of cross cutting concerns
Logging
Exception Handling
Logging
Synchronizing
Business Logic
Synchronizing
Exception Handling
Business Logic
13. Dynamic Proxy
The Players:
java.lang.reflect.Proxy
- dynamic proxy-creation facility
Has methods to create a class that can work as a proxy
for another class.
Proxy instance provided, can implement the interfaces
implemented by the actual class; the class for which it
acts as a proxy.
java.lang.reflect.InvocationHandler - Each proxy instance has an associated invocation handler.
When a method is invoked on a proxy instance, the method invocation is encoded and dispatched to the invoke
method of its invocation handler.
Object InvocationHandler.invoke(Object proxy, Method method, Object[] args)
throws Throwable
method - the Method instance corresponding to the interface method invoked on the proxy instance.
args - an array of objects containing the values of the arguments passed in the method invocation on the proxy
instance.
=> Proxy can provide a Class or an instance that implements a given set of interfaces, of which every method call
is intercepted and provided with an opportunity to do some something extra..
14. Call Stacks & Stack Frames 1 of 2
- Each thread of execution has a call stack consisting of stack frames.
- Each frame in the call stack represents a method call.
Call Stack
Stack Frames
- Stack frames contain information pertinent to the
associated method.
- Each new method call corresponds to a new stack frame.
- Frame at the bottom of a call stack will be
main() or run()
StackTraceElement[] java.lang.Throwable.getStackTrace()
- new Throwable().getStackTrace()
- returns StackFrames Since the main() or run()
StackTraceElement provides following details:
File name, Line number, Class name, method name
15. Call Stacks & Stack Frames 2 of 2
Uses:
1.
Logging
2.
Security - Based on the caller's package or class access can be denied
new Throwable().getStackTrace()[1] = ?
Sample Usage:
main()
method1()
method2()
method3()
{
{
{
{
#16
m1(); #27
}
}
m2(); #38 m3() ; #56 StackTraceElement[] elt = new Throwable().getStackTrace();
}
}
Output:
elt[0] = Class : call.stack.introspection.CallStack1SampleApp File : CallStack1SampleApp.java Line : 56 Method :
method3
elt[1] = Class : call.stack.introspection.CallStack1SampleApp File : CallStack1SampleApp.java Line : 37 Method :
method2
elt[2] = Class : call.stack.introspection.CallStack1SampleApp File : CallStack1SampleApp.java Line : 27 Method :
method1
elt[3] = Class : call.stack.introspection.CallStack1SampleApp File : CallStack1SampleApp.java Line : 16 Method :
main
16. Is performance overstated?
"We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil. Yet
we should not pass up our opportunities in that critical 3%. A good programmer will not be lulled into complacency by
such reasoning, he will be wise to look carefully at the critical code; but only after that code has been identified"
— Donald Knuth
Reflective flexibility = Binding the names on run time..
●
Categorizing performance impact:
○ Construction overhead - One time
■ Dependency injection (Spring)
■ Dynamic Proxy for specific methods
■ Reflective code generation
○
○
●
Execution Overhead
■ Method.invoke()
■ Forwarding method through a proxy.
Granularity Overhead
■ Method.invoke() end up doing a lot of unnecessary checks
Granularity overheads should be narrowed whenever possible.
17. Through the JDKs..
●
JDK 1.0
○
●
Class, Object, ClassLoader
JDK1.1
○
●
java.lang.reflect, Field, Method, Constructor
JDK1.2
○
●
●
●
●
●
AccessibleObject, ReflectPermission
JDK1.3
○ Proxy, InvocationHandler, UndeclaredThrowableException,
InvocationTargetException
JDK1.4
○ StackTraceElement
JDK1.5
○ Generics support, Annotations
JDK1.6
○ 'Generified' some methods in Class: getInterfaces(), getClasses() , etc
○ The final parameter of Array.newInstance(Class, int...) is of variable arity.
JDK7
○ ReflectiveOperationException - A "Common superclass of exceptions thrown by reflective
operations in core reflection.
18. References
'Standing on the shoulders of giants'
Java Reflection in Action (In Action series) - Ira Forman and Nate Forman
http://www.javaworld.com/javaworld/jw-09-1997/jw-09-indepth.html?page=1
http://java.sun.com/developer/technicalArticles/ALT/Reflection/
http://docs.oracle.com/javase/tutorial/essential/environment/security.html
http://docs.oracle.com/javase/1.4.2/docs/guide/security/PolicyFiles.html
http://docs.oracle.com/javase/tutorial/reflect/TOC.html
http://java.sun.com/developer/technicalArticles/DynTypeLang/
http://stackoverflow.com/
