Transcript ECE 750 Topic 8 - University of Waterloo

ECE 750 Topic 8
Meta-programming languages,
systems, and applications
Load-time structural reflection in Java
– Shigeru Chiba, 2000
May 27, 2004
Shimin Li
University of waterloo
Paper Overview
Contents: Overview
• Existing reflection facilities in Java
- Standard Java Reflection API
- Extensions to the reflection ability of Java
• Javassist – an intercession extension to Java
- Implementation technique
- Introduction to load-time structural reflection
- Javassist API
• Three applications of Javassist
Contents: Detail
• Existing reflection facilities in Java
- Standard Java Reflection API
• Restricted to introspection
• Behavior reflection is very limited
- Extensions to Java Reflection API
• Enable restricted behavior reflection
• Modify runtime systems or compilers
• Use hook-insertion technique and metaobjects
• Runtime overheads can be low
• Limitation to some language extensions(e.g. BCA)
Contents: Detail
• Novel reflection architecture – Javassist
- An extension to the Java reflection API
• Enables structural reflection in Java at load time
- Is portable
• Runs with a standard JVM
- Provides source-level abstraction for programmers
• Can be used without knowledge of the Java byte code
- Never needs source code
• Performs structural reflection by instrumenting byte code of
a loaded class
Contents: Detail
• Javassist API
- Reification
• CtClass object and its reflective methods
- Introspection
• Almost compatible with the Java reflection API
- Alteration
• Three design goals and their achievements
- Adding a new member
• Two approaches for adding a new method to a class
Contents: Detail
• Javassist API cont'd
- Altering a method body
- Reflective class loader
• Allows a loaded program to control the class loading
- Three ways to use Javassist
• With a user class loader
• With a web server
• Off line
Contents: Detail
• Application examples of Javasssist
- Binary Code Adaptation(BCA)
- Behavioral reflection
- Remote method invocation
• Generating stub code for RMI
Related Work
• Reflection in Java
- Behavioral reflection examples: MetaXa, Kava, Java reflection API
- Structural reflection example: Linguistic reflection
• Compile-time metaobject protocol
- Source-code basis example: OpenJava
• Byte code translator
- Examples: JOIE, JavaClass API
• Others
- OpenJIT, CLOS MOP, BCA
Contributions and Novelties
• Architecture for performing structural refection in
Java based on byte code
- No need for source code
- Used with a standard JVM and Java compiler
- Source-level abstraction
• Practical structural reflection API
• Three application examples
- Show implementing some language extensions with Javassist
is much easier than other reflection architectures
What I like
• Javassist does not break security guarantees given
by Java
• Javassist is easy to used
- Without knowledge of the Java byte code
- Can be used with a user class loader, web server, and offline
• Javassist architecture can be applied to other
object-oriented languages
- as long as the compiled binary includes enough symbolic information
to construct a class object
Questions
Question 1
What is structural reflection?
Answer
Structural reflection is the ability to allow
a program to alter the definitions of data
structures such as classes and methods.
Question 2
Some languages(e.g. Smalltalk,CLOS)
implement structural reflection with support
mechanisms embedded in runtime systems.
What problems would the application of this
implementation technique to Java cause?
Answer
• Invalidation of portability of Java due to
modification of JVM
• Performance degradation of JVM due to
difficulty to employ optimization at runtime
• Extra runtime type checks or restrictions on
the structural reflection
• Less security
Question 3
What is load-time structural reflection?
Answer
• Structural reflection is performed only
before a program is loaded into a runtime
system, that is, at load time.
Question 4
How does Javassist perform structural
reflection?
Answer
• Javassist implementation based on loadtime structural reflection architecture
• Javassist performs structural reflection by
translating alteration into equivalent
bytecode transformation of the class files
• After the transformation, the modified class
files are loaded into the JVM and then no
alteration are allowed after that
Question 5
How does Javassist reify the class needed to
be reflected?
Answer
• Javassist creates a CtClass(compile-time
class) object
• CtClass represents the byte code of the class
need to be reflected
• Then the class is accessible from a program
by invocating the methods on CtClass
object
Question 6
How is the introspecting facilities of
Javassist?
Answer
• The introspection part of Javassist API is
compatible with the Java reflection API
• But, Javassist does not provide methods for
creating an instance or invoking a method
Question 7
What is the main features of the alteration
methods in Javassist API?
Answer
• Provides source-level abstraction for users
- Approach to specify a method body is in source level
• Executes structural reflection efficiently
- Copy pre-compiled method body from a class to another
• Performs structural reflection in a safe manner in
terms of types
- No methods to remove a method or a field from a class
- New superclass must be a subclass of the original superclass
- No methods to change the parameters of a method
Question 8
What is the difference between the class
loader provided by Javassist and the Java’s
standard class loader?
Answer
• The Javassist class loader allows a loaded program
to control the class loading (see Figure 1)
- If a program is loaded by Javassist’s class loader L and it
includes a class C, then it can intercept the loading of C
by L to self-reflectively modify the byte code of C
- For avoiding infinite recursion, while the loading of a
class is intercepted, further interception is not allowed
∙
Java’s class loader never allows this self-reflective
class loading
Question 9
Why Javassist does not break security
guarantees given by Java?
Answer
• Javassist uses the standard JVM
• The classes processed by Javassist are
subject to the bytecode verifier and the
SecurityManager of Java
Question 10
Can Javassist and OpenJava both deal with
syntax extensions of Java?
Answer
• OpenJava can deal with syntax extensions.
Because OpenJava is source-code basis. It reads
source code for creating an object representing a
class, a method, or a field, and alteration on the
object is translated into corresponding
transformation of the source code
• Javassist can not deal with syntax extensions since
Javassist is byte-code basis
Question 11
Can the architecture applied by Javassist be
applied to other object-oriented language?
Answer
• Yes, as long as the compiled binary program
contains enough symbolic information to
construct a class object.
• However the API needs to be individually
designed for each language to perform
structural reflection in a safe manner in
terms of types