How JaCoCo Can Reveal and Eliminate Zombie Code to Boost Backend Efficiency

1. Situation & Problem

Rapid requirement iteration causes business code to grow quickly. While developers may be proud of high code output, they often lack visibility into how much of that code is actually used by online customers, leading to hidden "zombie" code that inflates maintenance cost.

2. Why Zombie Code Appears

Initial product analysis missed certain business scenarios.

During development, functional direction deviated from the original plan.

After launch, external factors reduced customer traffic.

Other unspecified reasons.

3. Measures

Use JaCoCo (Java Code Coverage) to probe runtime execution, identify which classes and methods are never invoked, and then safely deactivate or delete the corresponding dead code.

4. Implementation Steps

4.1 Add JaCoCo Ant Dependency

<dependency>
    <groupId>org.jacoco</groupId>
    <artifactId>org.jacoco.ant</artifactId>
    <version>0.8.3</version>
</dependency>
<dependency>
    <groupId>org.apache.ant</groupId>
    <artifactId>ant</artifactId>
    <version>1.9.9</version>
</dependency>

4.2 Expose a REST Endpoint to Dump Coverage

@RestController
@RequestMapping("/coverage")
public class CoverageController {

    @PostMapping("dump")
    @NoCheckMenuPermission
    public Result<Boolean> dumpCoverageFile() {
        DumpTask dumpTask = new DumpTask();
        // dump file location
        dumpTask.setDestfile(new File("/export/Data/coverage/code-cover.exec"));
        // append on multiple dumps
        dumpTask.setAppend(true);
        // choose an idle port
        dumpTask.setPort(8840);
        // bind to localhost
        dumpTask.setAddress("127.0.0.1");
        dumpTask.execute();
        return Result.succeed(true);
    }
}

4.3 Embed JaCoCo Agent

Decompress the JaCoCo agent JAR to avoid involving operations personnel.

# Decompress dependency to obtain jacocoagent.jar
jar -xvf $BASEDIR/lib/org.jacoco.agent-0.8.3.jar

-javaagent:$BASEDIR/bin/jacocoagent.jar=includes=com.jdwl.*,output=tcpserver,port=8840,address=127.0.0.1 -Xverify:none

4.4 Reserve JDOS Resource

Configure a custom /export directory for coverage files and add a cleanup script.

#!/bin/bash
ls -lh /export | awk 'NR >1 {print}' | awk '{if ($9 != "Data") print $9}' | xargs -i /bin/rm -rf /export/{} > /dev/null 2>&1
ls -lh /export/Data | awk 'NR >1 {print}' | awk '{if ($9 != "jdos.jd.com" && $9 != "coverage") print $9}' | xargs -i /bin/rm -rf /export/Data/{} > /dev/null 2>&1

4.5 Download Coverage File

The generated file is located at /export/Data/coverage/code-cover.exec.

4.6 Analyze Coverage

Open IntelliJ IDEA → Run → Show Coverage Data, select the .exec file, and view green (covered) versus red (uncovered) lines.

5. Efficiency Gains

5.1 Faster Delivery

After removing dead code, the average delivery cycle dropped from about 15 days to 12 days (January 2023).

5.2 Reduced Development Phase Duration

Development stage time shortened from 4.54 days to 3.11 days, a ~31 % reduction.

5.3 Human Efficiency

Removing zombie code lowers cyclomatic complexity and duplicate blocks (from ~31 to ~27), reducing cognitive load and maintenance cost.

Per‑person demand throughput increased from ~1.5 to ~2.5 after January 2023.

5.4 Process Quality

Automation bugs fell from 11/month to 5/month, and unit test coverage rose from 51.33 % to 52.28 %.

6. Brief Summary

Continuous requirement iteration inevitably expands codebases, and without runtime visibility, dead code accumulates, raising operational costs. Leveraging JaCoCo probes to measure actual code execution enables targeted deprecation, shrinking the codebase, lowering developers’ cognitive load, and improving delivery speed, human efficiency, and overall software quality.

References

JaCoCo Documentation – https://www.jacoco.org/jacoco/trunk/doc/index.html

javaagent Guide – https://www.cnblogs.com/rickiyang/p/11368932.html

Practical JaCoCo Coverage on Services – https://www.cnblogs.com/ccoder/p/15369719.html#4946710

Bytecode Manipulation with ASM & Javassist – https://newrelic.com/blog/best-practices/java-performance-monitoring