MS_EXPOSE_REP triggered for avro's BinaryMessageDecoder

[hejcz]

I believe this is false positive. Why is BinaryMessageDecoder treated as an array?

MS: Public static method may expose internal representation by returning array (MS_EXPOSE_REP)

A public static method returns a reference to an array that is part of the static state of the class. Any code that calls this method can freely modify the underlying array. One fix is to return a copy of the array.

[INFO] --- spotbugs-maven-plugin:4.3.0:check (default) @ spotbugs-bug ---
[INFO] BugInstance size is 1
[INFO] Error size is 0
[INFO] Total bugs: 1
[ERROR] Medium: Public static io.github.hejcz.TestClass.getDecoder() may expose internal representation by returning TestClass.DECODER [io.github.hejcz.TestClass] At TestClass.java:[line 16] MS_EXPOSE_REP

package io.github.hejcz;

import org.apache.avro.message.BinaryMessageDecoder;
import org.apache.avro.specific.SpecificData;

public class TestClass {

    private static SpecificData MODEL$ = new SpecificData();

    public static final org.apache.avro.Schema SCHEMA$ = new org.apache.avro.Schema.Parser().parse("");

    private static final BinaryMessageDecoder<TestClass> DECODER =
            new BinaryMessageDecoder<TestClass>(MODEL$, SCHEMA$);

    public static BinaryMessageDecoder<TestClass> getDecoder() {
        return DECODER; // spotbugs detects bug in this line
    }

}

<plugin>
    <groupId>com.github.spotbugs</groupId>
    <artifactId>spotbugs-maven-plugin</artifactId>
    <version>4.3.0</version>
    <configuration>
        <failOnError>true</failOnError>
        <includeTests>false</includeTests>
    </configuration>
    <executions>
        <execution>
            <phase>compile</phase>
            <goals>
                <goal>check</goal>
            </goals>
        </execution>
    </executions>
</plugin>

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[hejcz]

I tried to create an example but the test seems to pass. Maybe there is some problem with a plugin or it is fixed on master or the problem is java 16 we use.

An attempt to reproduce the problem: hejcz@1be7ac0
Project that actually fails: https://github.com/hejcz/spotbugs-avro-fp

[kuzmeo]

Issue is reproducible if you set report level to "medium" and when your method returns some private static instance of class, that has some fields.
I think, I got what is meant under MS_EXPOSE_REP warning: if you return some static private instance of class, someone could set some fields in that instance, so it will be changed, but it should not be.

[gtoison]

BinaryMessageDecoder is deemed mutable because it has the addSchema() method, see #1797
The misleading message (only mentionning arrays) was fixed in #2535