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Tutorial / lab 2: Code instrumentation

Tutorial / lab 2: Code instrumentation. Goals of this session: Create some tests for a Java class. Measure the code coverage. Code Instrumentation Tools. Tools that modify production code for various purposes during code execution: Debuggers

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Tutorial / lab 2: Code instrumentation

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  1. Tutorial / lab 2: Code instrumentation • Goals of this session: • Create some tests for a Java class. • Measure the code coverage.

  2. Code Instrumentation Tools • Tools that modify production code for various purposes during code execution: • Debuggers • Add information to allow for break points or step-by-step execution. • Coverage measurement • Report on the number of times various code elements have been executed. • Profilers • Measure amount of time spent in various parts of the code.

  3. Coverage tools • Program is typically compiled with special options, to add extra source or object code. • Additional data structures, such as a flow graph, may also be created. • Program is run, possibly via test cases • During execution, information is accumulated and written to an output file. • Post-processing phase: • User report is generated from output file.

  4. How to measure coverage? • Instrument the source code before compilation • Instrument the virtual machine or object code. • The Apache Byte Code Engineering Library (BCEL) is used by several Java coverage tools • http://jakarta.apache.org/bcel • Use a customized virtual machine.

  5. Source code instrumentation • Create a copy of the source code, instrument the copy, and compile the instrumented copy to .class files. • This is the approach used by the Clover code coverage tool. • Typical method: • Set up an array of counters. • Insert source code at points of interest that increment specific counters. • Coverage tool calls main() method • After main method returns, dump values of counters.

  6. Source code before instrumentation public final void add( final MatrixRow<T> newValues ) { if ( getNumColumns( ) == 0 ) { contents = newValues.copy( ).getContents( ); } else { contents.addAll( newValues.copy( ).getContents( ) ); } } • A counter is inserted for each method, statement, and branch. • Statement counters are before each statement, in case the statement throws an exception. • Branch counters need to identify true and false branches taken.

  7. Same code after instrumentation public final void add( final MatrixRow<T> newValues ) { CLOVER.M[348]++; CLOVER.S[1814]++; if ( ( getNumColumns( )==0 && ++CLOVER.CT[338]!=0 | true ) || ( ++CLOVER.CF[338] == 0 & false ) ) { CLOVER.S[1815]++; contents = newValues.copy( ).getContents( ); } else { CLOVER.S[1816]++; contents.addAll( newValues.copy( ).getContents( ) ); } } statement counters branch counters method counter

  8. Counting branches • Count number of times that a branch condition evaluates to true and false. if ( ( aBoolean && ++CT[338]!=0 | true ) || ( ++CF[338] == 0 & false ) ) • If aBoolean is true, then the && is also evaluated, which will increment the CT counter. We want the expression’s overall evaluation to be true. • If aBoolean is false, then execution switches to the right side of the || clause. Increment the CF counter, but then ensure that the entire expression still evaluates to false to take the else branch. • Short-circuit evaluation means that as soon as the expression result can be determined, evaluation stops. This avoids: • Incrementing the true counter if aBoolean is false. • Incrementing the false counter if aBoolean is true.

  9. Analysis • Advantages: • Results are directly mapped to source code. • If a statement throws an exception, it will be counted as having been executed. • Disadvantages: • Requires keeping a second version of source code, and ensuring that original source is not over-written. • Instrumented code must not affect original code execution. • Management of .class files from compiling alternate source code. • Memory usage: for statement coverage alone, every statement requires one integer as a counter.

  10. Byte code instrumentation • Can be done in two ways: • Instrument object files in advance to produce a alternate version. • Example: create instrumented .class files for Java. • Instrument object files as they are used. • Example: create customized .class file loader that inserts the instrumentation at run time. • This is the approach used by the Emma code coverage tool • This is an open-source tool; see http://emma.sourceforge.net • Eclipse plug-in version: http://www.eclemma.org

  11. Analysis • Pre-instrumentation: • Advantages: • source code is not modified. • can be done as a batch process before execution • Disadvantages: • management of two .class file versions. • mapping of source code statements to/from object code • counting source code statements can be difficult • object code may need extra information such as special compiler options

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