ProtoFuzzer is a mutable class, even when seeded by a compile-time constant. We're trying to avoid the following pitfalls which can arise when assigning a ProtoFuzzer to a static field:

• Accidental state leakage between unit tests, causing non-deterministic or flaky behavior. In this scenario, it's recommended to instantiate a distinct ProtoFuzzer for each relevant unit test, possibly by using a @Before method.

 private ProtoFuzzer protoFuzzer;
 ...
  @Before
  public void setUp() {
    ...
    // Customize as appropriate
    protoFuzzer = ProtoFuzzer.newBuilder().setSeed(...).build();
    ...
  }

• If a static ProtoFuzzer is used to initialize other static fields, then this initialization process can have program-order dependency; for example, re-ordering two such initialized fields can cause their values to change. This problem can be avoided by using static builder methods to initialize.

private static final ProtoFuzzer protoFuzzer =
    ProtoFuzzer.newBuilder()
               .setSeed(...)
               .build();
...
// Re-ordering myFirstCustomProto and mySecondCustomProto can change their values!
private static final MyCustomProto myFirstCustomProto =
    protoFuzzer.makeMessageOfType(
      MyCustomProto.getDefaultInstance()
    );
private static final MyCustomProto mySecondCustomProto =
    protoFuzzer.makeMessageOfType(
      MyCustomProto.getDefaultInstance()
    );

Instead, create a static builder method and replace references to the static ProtoFuzzer field with calls to the builder:

private static final MyCustomProto myFirstCustomProto =
  buildMyCustomProtoFuzzer().makeMessageOfType(
    MyCustomProto.getDefaultInstance()
  );
private static final MyCustomProto mySecondCustomProto =
  buildMyCustomProtoFuzzer().makeMessageOfType(
    MyCustomProto.getDefaultInstance()
  );
...
private static ProtoFuzzer buildMyCustomProtoFuzzer() {
  // Customize as appropriate
  return ProtoFuzzer.newBuilder().setSeed(...).build();
}