On the Extent and Nature of Software Reuse in Open

Source Java ProjectsLars Heinemann, Florian Deissenboeck, Mario Gleirscher, Benjamin Hummel,

Maximilian IrlbeckTechnische Universität München

ICSR 2011, Pohang, Korea1

Software Reuse• Reuse of existing artifacts for constructing

new software

• Proven benefits

• Increased productivity

• Reduced time to market

• Improved quality

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• Tremendous reuse opportunities

• Class Libraries (e.g. Apache Commons)

• Frameworks (e.g. Eclipse: 40 MLOC)

• Open source code (Google Code Search: several GLOC)

• Internet serves as reuse repository

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Software Reuse

Research Problem• Unclear how software projects make use of

available reuse opportunities

• Lack of data on amount of reuse in software projects

• Assessing success of software reuse difficult

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Contribution• Empirical knowledge about extent and

nature of software reuse in OSS

• Quantitative data on software reuse in 20 open source projects

• Substantiates discussion of success/failure of software reuse

• Provides practioners with benchmark

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Terms• Software reuse: Using code developed by

third parties (excluding OS/platform)

• White-box reuse: Code incorporated in source form (internals exposed, potentially modified)

• Black-box reuse: Code incorporated in binary form (internals hidden, no modifications)

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Study Design (GQM)

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We analyze open source projects

for the purpose of understanding the state of the practice in

software reuse with respect to its

extent and nature from the viewpoint of the

developers and maintainers in the context of

Java open source software.

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Question Metric

RQ1: Do open source projects reuse software?

existence of software reuse

RQ 2: How much white-box reuse occurs?white-box reuse

rate

RQ 3: How much black-box reuse occurs?black-box reuse

rate

Study Design (GQM)

Reuse Rate

Reused source code [LOC]Overall source code [LOC]

White-box

Reused binary code [bytes]

Overall binary code [bytes]Black-box

Reused code

Project‘s own codeOverall code ofsoftware system

Study Objects• 20 Java projects from

• Criteria: Production/Stable, Standalone app, pure Java, Java SE platform, source download available

• All among 50 most downloaded

• sourcecode size: 0.4 to 790 kLOC, bytecode size: 17 to 22,761 KB

• Test code excluded with heuristics (e.g. folders named test/tests)

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Study Implementation

• White-box reuse = copied code

• Can be detected automatically by clone detectors

• Clone detection against 22 commonly used Java libraries (~ 6MLOC)

• Detection of reuse of statement sequences with > 15 statements

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a) Detecting white-box reuse

Study Implementation

• In addition: manual inspection of source directory tree

• Clues: file/package names

• Source of files identified via header comments/web search

• Detection of reuse of whole files/directories, not limited to fixed set of libraries

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a) Detecting white-box reuse

Study Implementation

• Byte-code based static analysis

• Aggregates byte code size of all library types referenced by project‘s source code

• Traverses type dependency graph using Java Constant Pool (type usages and method calls)

• Includes transitive dependencies

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b) Detecting black-box reuse

Study Implementation

• Although not covered by reuse definition, potential variations in use of Java API interesting

• Black-box reuse baseline of empty Java program: 5 MB (2,082 types)

• Object → Class → ClassLoader ... (Reflection API / Collections API)

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b) Detecting black-box reuse

Results RQ 1

• 18 of the 20 projects (90%) reuse software from third parties

• Exceptions: HSQLDB (relational database engine), Youtube Downloader (video download utility)

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Do open source projects reuse software?

Results RQ 2

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How much white-box reuse occurs?

• Clone detection found 791 clones, 11,701 copied LOC in 7 study objects

• Clones found: complete files with minor modifications (e.g. different version)

• Manual inspection found additionally whole copied libraries in 4 study objects

• Overall: white-box reuse found for 9 of 20 projects

• Reuse rates: 0% - 10%

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Results RQ 3

Absolute bytecode size distribution (MB)

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How much black-box reuse occurs?

3rd party: 0 - 42 MBJava API: 13 - 17 MB

Relative bytecode size distribution (%)

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Results RQ 3How much black-box reuse occurs?

3rd party: 0 - 62%Java API: 23 - 99%Combined: 41 - 99%

Relative bytecode size distribution (%) without Java API

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Results RQ 3How much black-box reuse occurs?

Discussion

• Software reuse common among Java OSS

• On average: high black-box reuse rates

• Expected to have significant impact on development effort

• Black-box reuse rates considerably varying

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a) Extent of reuse

Discussion

• Lee&Litecky found a negative influence of project size on reuse rate (survey of 500 Ada professionals)

• Without Java API: Spearman correlation of 0.05 (two tailed p-value 0.83)

• With Java API: Spearman -0.93 (p-value < 0.0001) → significant and strong negative correlation

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b) Influence of project size on reuse rate

Discussion

• Categorization of reused libraries (e.g. networking, text/xml, rich client platforms)

• No predominant category found

• Nearly all projects reuse software from more than one category

• No significant insights, except reuse diverse w.r.t. types of functionality

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c) Types of reused functionality

Threats to internal validity

• False-positives from clone detection

• mitigated by manual inspection of results

• Unclear if code was copied into study objects or from them

• mitigated by manual inspection

• Black-box analysis considers a whole class as the element of reuse

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a) overestimation of reuse

Threats to internal validity

• Fixed set of libraries in clone detection

• False-negatives in clone detection

• Manual inspection for copied code inherently incomplete

• Black-box analyses misses calls via reflection, boundaries by Java interfaces

• Other forms of component interaction

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a) underestimation of reuse

Threats to external validity• Unclear how representative study objects

are for all Java OSS

• Transferability to other PL or commercial development unclear

• Impact of PL is expected to be high

• Availability of reusable code depends on PL (e.g. Java vs. COBOL)

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Conclusions• Early visions of development by plugging

reusable components not realistic

• But: Reuse in form of libraries common in Java OSS

• High black-box reuse rates (9 of 20 projects > 50%)

• Availability of reusable functionality well-established for Java platform

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Future Work• Other programming ecosystems

• Legacy programming languages, e.g. COBOL

• Scripting languages, e.g. Python

• Commercial software development environments

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Thank you.Questions?

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