Reading Class Files

OPAL provides multiple different representations for Java class files to support different use cases. Next, we will discuss how to directly read Java class files and how to create the different representations.

The following examples expect that you have checked out OPAL, and that you started sbt in OPAL's main folder. After that, you have changed to the project project OPAL-DeveloperTools and started the console. Alternatively to checking out OPAL and building OPAL on your own, you can use our preconfigured Docker Container.

A growing number of small code snippets that demonstrate various aspects of the API can be found here.

Bare Bones 1:1 Representation of Java Class Files

A bit by bit representation is provided by the bytecode disassembler sub project. In this representation, the constant pool is kept and all other elements (e.g., names of classes, methods and fields, but also constant values etc.) use int based references to the constant pool to refer to the respective values. A single class file can trivially be loaded using:

import java.io.{DataInputStream, FileInputStream}
import org.opalj.io.process
import org.opalj.da.ClassFile
val classFileName = "OPAL/ai/target/scala-2.11/classes/org/opalj/ai/AI.class"
val cfs : List[ClassFile] =
    process(new DataInputStream(new FileInputStream(classFileName))){ in =>
        org.opalj.da.ClassFileReader.ClassFile(in)
    }

When you use this representation, the returned list always contains a single class file object – which you can get by calling head on the returned list.

Using this representation is very, very fast and makes it, e.g., easily possible to perform some simple method based slicing or to create an HTML representation (by calling toXHTML). The loaded class/transformed class can be written to a class file using org.opalj.bc.Assembler(<da.ClassFile object>)

Object-Oriented Representation of Java Class Files

In most cases, an explicit representation of the constant pool actually complicates the implementation of static analyses. To avoid that you have to deal with the constant pool, OPAL provides a standard object oriented representation that suits many needs. This representation is still stack based and, therefore, the operand stack is still present. It, nevertheless, often strikes a nice balance between performance, memory usage and convenience and, therefore, many analyses that are part of OPAL use this representation. In general, a list of class files is returned to support class file transformations while the class file is loaded. For example – if configured – invokedynamic instructions, which are, e.g., created by Java compilers when closures are used in Java code, will automatically be transformed to standard (pre Java 7) classes and method calls to faciliate subsequent analyses. In this case, a class is generated that will capture the closure's call state and the invokedynamic instruction will be replaced by a call to the generated class' factory method; this class serves a similar purpose as the call-site object that would be created by the JVM at runtime.

import java.io.{DataInputStream, FileInputStream}
import org.opalj.io.process
import org.opalj.br.ClassFile // "br" instead of "da"
val classFileName = "OPAL/ai/target/scala-2.11/classes/org/opalj/ai/AI.class"
val cfs : List[ClassFile] =
    process(new DataInputStream(new FileInputStream(classFileName))){ in =>
        org.opalj.br.reader.Java8Framework.ClassFile(in)
    }

Representing entire Java Projects

Instead of reading the class files on your own it is also possible to directly create a Project which also directly makes the class hierarchy available and offers many methods related to resolving method calls and the like. A Project is usually at the core of implementing static analyses on top of OPAL. To read more about it go here.

3-Address Code Representation

On top/based on the object oriented representation OPAL provides a third representation based on 3-address code/quadruples in single static assignment (SSA) form. This representation is directly made available by a Project on-demand. To read more about it go here.