Advanced Reflection in Java

SINF2335:Prog.Paradigms:
Theory,Pract.andApplic.
Lecture 10 : 
Advanced Reflection in Java
LSINF 2335
Programming Paradigms
Prof. Kim Mens
UCL / EPL / INGI
 
(Slides partly based on chapter 5 of
the book “Java Reflection in Action”
and on slides by Pr. Walter Cazzola)

Advanced reflective features
■ Dynamic proxies
– Proxy
– InvocationHandler
■ Call stack introspection
– Throwable
– StackTraceElement
■ Instrumentation
– java.lang.instrument
■ Conclusion
2

Proxy Design Pattern
3
■ The proxy pattern defines a proxy as a surrogate
for another object to control access to it
– a proxy keeps a reference to the real object,
– is substitutable with it (identical interface)
– and delegates requests to it
■ Typical examples of usage of proxies:
– local representation of remote objects
– delay of expensive operations
– access protection for secure objects
– ...
Client
action()
Subject
action()
Proxy
action()
RealSubject
delegate

Dynamic proxies
■ To easily implement proxies, Java 1.3 introduced
the Dynamic Proxy API
■ A dynamic proxy requires
– an instance of the Proxy class
– a proxy interface
• this is the interface that is implemented by the proxy class
• interestingly, one proxy can implement multiple interfaces
– each proxy instance has an InvocationHandler
4Proxy
InvocationHandler
delegates to
IProxy
implements
ProxyHandler
implements

InvocationHandler
■ Each proxy instance has an InvocationHandler
– The invocation handler determines how to treat messages
that are sent to the proxy instance
– 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 invoke(Object proxy, Method m, Object[] args)
5
Proxy
invoke(Object, Method, Object[]) : Object
<<interface>>
InvocationHandlerdelegates to

Theory,Pract.andApplic. Example: an InvocationHandler
to trace method calls
6
public class TraceHandler implements InvocationHandler {
private Object baseObject; // the real object the proxy delegates to
public TraceHandler(Object base) {
baseObject = base;
}
public Object invoke(Object proxy, Method m, Object[] args) {
Object result = null; // result to be returned by the method
try {
System.out.println("before " + m.getName());
result = m.invoke(baseObject, args);
System.out.println("after " + m.getName());
}
catch (Exception e) { 
e.printStackTrace();
}
return result;
} 
}

Proxy class
■ Provides static methods for creating dynamic proxy classes and
instances
■ Is also the superclass of all dynamic proxy classes created by those
methods
■ To create a proxy for some class Foo 
InvocationHandler handler = new MyInvocationHandler(...);
Class proxyClass = Proxy.getProxyClass(
Foo.class.getClassLoader(), new Class[] { Foo.class });
Foo f = (Foo) proxyClass.
getConstructor(new Class[] { InvocationHandler.class }).
newInstance(new Object[] { handler });
■ or more simply: 
Foo f = (Foo) Proxy. 
newProxyInstance( Foo.class.getClassLoader(),
new Class[] { Foo.class },
handler );
7

Theory,Pract.andApplic. Example: a Proxy that traces
method calls
8
public class Component1 implements IComponent {
Color myColor;
public Color getColor() {
System.out.println("Inside the getColor() method of Component1.");
return myColor;
}
public void setColor(Color color) {
myColor = color;
System.out.println("The color of component 1 has been set.");
}
}
public interface IComponent {
public Color getColor();
public void setColor(Color color);
}
interface
shared by real
object and proxy
real object to
which proxy will
delegate

Theory,Pract.andApplic. Example: a Proxy that traces
method calls
9
IComponent c1 = new Component1(new Color(0));
InvocationHandler th = new TraceHandler(c1);
IComponent proxy = (IComponent) Proxy.newProxyInstance(
c1.getClass().getClassLoader(), 
c1.getClass().getInterfaces(),
th);
/* standard call */
c1.getColor();
/* traced call */
proxy.getColor();

How Java creates a proxy
10
Bibliography Examples: Tracing and Proxy Chaining.
Java’s Dynamic Proxy.
How Java Creates a Proxy.
extends
implements
implements
implements
$IPoint
InvocationHandlerIPoint Proxy
Point TraceHandler
Classes
Interfaces
p thpth
- int x
- int y
+ getX(): int
+ getY(): int
+ getX(): int
+ getY(): int
+ getX(): int
+ getY(): int
- Object base
+ invoke(): Object
- InvocationHandler i
+ invoke(): Object
Generated
on-the-ﬂy

■ Dynamic proxies
– Proxy
– Throwable
■ Instrumentation
■ Conclusion
11

Call Stack Introspection
■ State introspection
■ Call stack
■ Reifying the call stack
– Throwable
■ Examples:
– printing the call stack
– show warnings for unimplemented methods
12

State Introspection
13
■ Introspection is not only application structure
introspection
■ Information about the program execution can be
introspected as well
– the execution state
– the call stack
■ Each thread has a call stack consisting of stack
frames
■ Call stack introspection allows a thread to examine
its context
– the execution trace and the current frame

Call Stack
1. package reflectionexample;
2.
3. public class Example {
4.
5. public static void m1() {
6. m2();
7. }
8.
10. m3();
11. }
12.
14. // do something
15. }
16.
17. public static void main(String[] args) {
18. m1();
19. }
20.
21. }
class: Example
method: main
line: 18
class: Example
method: m1
line: 6
Call Stack:
class: Example
method: m2
line: 10
class: Example
method: m3
line: 14
(m3)

Reifying the call stack :
Throwable
15
■ In Java there is no accessible CallStack meta-object
■ But...
– When an instance of Throwable is created, the call stack is
saved as an array of StackTraceElement
– By writing: new Throwable().getStackTrace()
– This gives us access to a representation of the call stack at
the moment when Throwable was created.
– The getStackTrace() method returns the current call stack
as an array of StackTraceElement.
• The first frame (position 0) is the current frame
■ Only introspection

Example 1: print the call stack
16
public class CallStackExample1 {
public static void m1() {
m2();
}
m3();
}
StackTraceElement[] stack = new Throwable().getStackTrace();
for (int i = 0; i < stack.length; i++) {
System.out.println(stack[i]);
}
}
public static void main(String[] args) {
m1();
}

Example 2: show warnings for
unimplemented methods
■ How to create an auxiliary method to be used as
placeholder for unimplemented methods?
– I have a method todo() remaining to be implemented
– I provide the following dummy implementation
– When calling that method todo() I want to get a warning:
– as well as a warning message on the Console:
Warning:
reflectionexample.CallStackExample2.todo(CallStackExample2.java:8)
() : this method remains to be implemented
public static void todo() {
toBeImplemented();
}

18
Example 2: show warnings for
methods to be implemented
public static void todo() {
toBeImplemented();
}
public static void toBeImplemented() {
// Get the stack element referring to the method calling this one
StackTraceElement el = new Throwable().getStackTrace()[1];
// Show a dialog box with information on the method remaining to be
implemented
String msg = el.toString() + "() : this method remains to be implemented";
// Show this message in a dialog box
JOptionPane.showMessageDialog(null, msg, "Erreur", JOptionPane.ERROR_MESSAGE);
// Print this warning on the console
System.err.println("Warning: " + msg);
}
public static void main(String[] args) {
todo();
}
}

Reifying the call stack :
StackTraceElement
■ From a frame we can get:
– getFileName() : the filename containing the execution point
– getLineNumber() : the line number where the call occurs
– getClassName() : the name of the class containing the 
execution point
– getMethodName() : the name of the method containing the  
execution point
19
“myPackage.Example.m3(Example.java:14)”
StackTraceElement:
class: Example
method: m3
line: 14
filename: Example.java

■ Dynamic proxies
– Proxy
– Throwable
■ Instrumentation
■ Conclusion
20

Suppose we want to...
1. Instrument a Java program to print a message on
the console whenever a class is loaded by the JVM
2. Instrument a Java program to print all messages
being sent while the program is running
3. Replace the definition of a class at runtime
– even for classes with currently active instances
– for example, to change the behaviour of some messages
understood by those instances
21
... then Java instrumentation may be your answer

Theory,Pract.andApplic. Java Instrumentation
(goal)
22
■ java.lang.instrument
– Provides services that allow Java programming language
agents to instruments programs running on the JVM.
– This instrumentation is achieved by modifying byte-code.
– Allows you to create agents, that run embedded in a JVM
and intercept the classloading process, to
• monitor the classloading process
• modify the bytecode of classes
■ Can be combined with dedicated libraries for Java
bytecode manipulation, like Javassist or BCEL.

(mechanics)
23
To implement an agent that intercepts classloading
you need to define
– a class implementing the premain method 
 
public static void premain(String agentArguments,  
Instrumentation instrumentation) { ... }
– a class implementing a transformer (which describes how the
bytecode should be transformed) 
 
public class SomeTransformer implements ClassFileTransformer 
public byte[] transform(ClassLoader loader,  
String className, Class redefiningClass, 
ProtectionDomain domain, byte[] bytes) 
throws IllegalClassFormatException { ... }
– you can put both methods in a single class

(mechanics)
24
– The transform method receives information on the class to be
loaded and can modify its bytecode. 
 
public byte[] transform(ClassLoader loader,  
String className, Class redefiningClass, 
ProtectionDomain domain, byte[] bytes) 
• returns null if there's no modification, else returns the new
bytecode
• to modify the bytecode you can use a specialised library like
Javassist 
– The premain method should add the transformer to the agent: 
 
public static void premain(String agentArguments,  
Instrumentation instrumentation) { 
instrumentation.addTransformer(new SomeTransformer()); }

(mechanics)
25
– To plug the agent into the JVM and execute it, you need to put
it inside a jar: 
jar cvfm MyAgent.jar ManifestFile.txt 
MyAgent.class SomeTransformer.class
– The manifest file for this jar has to declare the premain class: 
Premain-Class: MyAgent 
If the agent modifies the bytecode, this also has to be
declared in the manifest file: 
Can-Redefine-Classes: true
– Finally, to instrument a Java program you need to add the
javaagent parameter when you execute the JVM:
> java -javaagent:MyAgent.jar MyProgram

Examples
26

Example 1: Instrument Java code
27
/**
* Just prints a message to stdout before each Class is loaded
* > java -javaagent:LogClassLoad.jar <the_Main_Class_to_execute>
*/
public class LogClassLoad implements ClassFileTransformer {
public static void premain(String options, Instrumentation ins) {
ins.addTransformer(new LogClassLoad());
}
public byte[] transform(ClassLoader loader, String className, Class cBR,
java.security.ProtectionDomain pD, byte[] classfileBuffer)
throws IllegalClassFormatException {
try {
System.out.println("LOADING: " + className);
}
catch (Throwable exc) {
System.err.println(exc);
}
return null; // For this first example no transformation is required :
// we are only logging when classes are loaded
}
}

Example 1: Instrument Java code
28

Examples
29

Example 2: Print all messages
30
public class TraceMethodCall implements ClassFileTransformer {
public static void premain(String options, Instrumentation ins) {
ins.addTransformer(new TraceMethodCall());
}
public byte[] transform(ClassLoader loader, String className, Class cBR,
java.security.ProtectionDomain pD, byte[] classfileBuffer)
throws IllegalClassFormatException {
try {
if(isSystemClass(className)) return null;
ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get(className);
CtMethod[] allmethods = cc.getMethods();
for(CtMethod method : allmethods) {
try {
method.insertBefore("TraceMethodCall.trace();");
} catch (Exception e) { }
}
return cc.toBytecode();
}
catch (Throwable exc) { }
return null; // No transformation required
}
for(CtMethod method : allmethods) {
try {
method.insertBefore("TraceMethodCall.trace();");
} catch (Exception e) { }
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;

31
public class TraceMethodCall implements ClassFileTransformer {
...
/** Check if a class is a system class, based on the package name. **/
public static boolean isSystemClass(String className) { ... }
public static boolean isDottedSystemClass(String className) { ... }
...
public static void trace() {
Throwable thw = new Throwable();
if(thw.getStackTrace().length > 2) {
StackTraceElement stackTo = thw.getStackTrace()[1];
StackTraceElement stackFrom = thw.getStackTrace()[2];
if(!isDottedSystemClass(stackFrom.getClassName())) {
System.out.println(""" + stackFrom.getClassName() + "."  
+ stackFrom.getMethodName() + "" -> " + """  
+ stackTo.getClassName() + "." + stackTo.getMethodName() + """);
}
}
}
}

32
package reflectionexample;
public static void m1() { m2(); }
public static void m2() { m3(); }
public static void m3() { }
public static void main(String[] args) { m1(); }
}

Examples
33

Theory,Pract.andApplic. Example 2: Replacing a class
(goal)
34
/**
* This modified class replaces the original class at run-time.
* The modified class has the same name but is located in a subdirectory.
* It is a clone of the original class where one method was changed.
* Instances of this class will react differently to those messages.
*/
public class SwappedClass {
public void message() {
System.out.println("I am the MODIFIED SwapedClass");
}
}
/**
* This is the original class that will be replaced by a modified copy at run-time.
* The modified copy of this class is located in the "modif" subdirectory.
*/
public class SwappedClass {
public void message() {
System.out.println("I am the original SwapedClass");
}
}
original
modifie

(mechanics)
■ to dynamically replace a class execute this code:
35
import java.lang.instrument.Instrumentation;
public class SwapClass {
private static Instrumentation instCopy;
public static void premain(String options, Instrumentation inst) {
instCopy = inst;
}
public static Instrumentation getInstrumentation() {
return instCopy;
}
}
SwapClass.getInstrumentation().redefineClasses(...)

(code)
36
public class SwapTest {
public static void main (String[] args) {
try {
// Create an instance of the class that will be modified
SwappedClass swapped = new SwappedClass();
// Send a message to the instance; the original message should be displayed
swapped.message();
// Now replace the class by a modified version
// 1. read the class definition from file
FileChannel fc =
new FileInputStream(new File("modif/SwappedClass.class")).getChannel();
ByteBuffer buf = fc.map(FileChannel.MapMode.READ_ONLY, 0, (int)fc.size());
byte[] classBuffer = new byte[buf.capacity()];
buf.get(classBuffer, 0, classBuffer.length);
// 2. use instrumentation API to replace the old class’s definition by the new one
SwapClass.getInstrumentation().redefineClasses(
new ClassDefinition[] {
new ClassDefinition(swapped.getClass(), classBuffer)});
// Send a message to the old instance; the MODIFIED message will be displayed
swapped.message();
}
catch (Exception e){
System.out.println(e);
}
}
}
SwapClass.getInstrumentation().redefineClasses(
new ClassDefinition[] {
new ClassDefinition(swapped.getClass(), classBuffer)});

(at work)
37

Reflection in Java: Conclusions
38
■ Benefits
– reflection in Java opens up the structure and the
execution trace of the program;
– the reflection API is (relatively) simple and quite
complete.
■ Drawbacks
– reflection in Java is (mostly) limited to introspection;
– there isn't a clear separation between base and meta-
level;
– reflection in Java has been proved inefficient

Advanced Reflection in Java

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