Why JaCoCo Can Break Your Java Service: ClassCastException Explained and Fixed

Problem Description

JaCoCo is a widely used open‑source coverage tool. After adding it to the test environment, the machine starts normally, but the order‑placement process throws an exception, blocking the workflow.

Removing the JaCoCo configuration, recompiling and redeploying restores normal order processing. The stack trace shows a ClassCastException caused by a failed type conversion: [Z cannot be cast to [Ljava.lang.Object;.

Stack Trace

java.lang.ClassCastException: [Z cannot be cast to [Ljava.lang.Object;
    at com.jd.**.TdeProxy.encryptObject(TdeProxy.java:93)
    at com.jd.**.TdeProxy.encryptObject(TdeProxy.java:133)
    at com.jd.**.TdeProxy.encryptObject(TdeProxy.java:90)
    ... (omitted)

Analysis

1. Error Code

The error occurs when the code attempts to cast a boolean[] (represented as [Z in JVM signatures) to an Object[]. This happens in the encryptObject method during array handling.

2. Analysis Path

The encryptObject method uses recursion to traverse collections. The problem lies in the element set that cannot be cast correctly, which points to the getDeclaredFieldsAll method that collects class fields.

public Field[] getDeclaredFieldsAll(Class clazz) {
    List<Field> fieldsList = new ArrayList<>();
    while (clazz != null) {
        Field[] declaredFields = clazz.getDeclaredFields();
        fieldsList.addAll(Arrays.asList(declaredFields));
        clazz = clazz.getSuperclass();
    }
    return fieldsList.toArray(new Field[fieldsList.size()]);
}

JaCoCo injects a synthetic field named $jacocoData (a boolean[]) into every class. When the reflection utility processes this field, the cast to Object[] fails, producing the observed exception.

3. Synthetic Fields and Methods

Synthetic members are generated by the Java compiler to support internal language features. For example, accessing a private field of an inner class from the outer class creates a synthetic accessor method.

public class Pack {
    public static void main(String[] args) {
        Pack.Goods goods = new Pack.Goods();
        System.out.println(goods.name);
    }
    private static class Goods {
        private String name = "手机";
    }
}

After compilation, the compiler adds a method like access$100 to allow the outer class to read the private name field. Such synthetic methods appear in bytecode but are not part of the source code.

4. JaCoCo Principle

JaCoCo uses ASM to insert probe booleans into bytecode. Each probe is a boolean stored in the generated $jacocoData array. The probes record whether a piece of code has been executed without altering program logic.

Solution

Filter out synthetic fields when gathering class fields. The Field.isSynthetic() method identifies compiler‑generated members.

List<Field> fieldsList = Arrays.stream(declaredFields)
    .filter(field -> !field.isSynthetic())
    .collect(Collectors.toList());

After applying this filter and redeploying with JaCoCo enabled, the order‑placement functionality works correctly.

Conclusion

This article records a troubleshooting case where adding JaCoCo introduced a runtime ClassCastException due to the synthetic $jacocoData field. Developers using reflection or annotation‑based tools should be aware of synthetic members and filter them to improve code robustness.