private com.google.protobuf.SingleFieldBuilderV3<
            com.google.devtools.cloudtrace.v2.TruncatableString,
            com.google.devtools.cloudtrace.v2.TruncatableString.Builder,
            com.google.devtools.cloudtrace.v2.TruncatableStringOrBuilder>
        getBuildIdFieldBuilder() {
      if (buildIdBuilder_ == null) {
        buildIdBuilder_ =
            new com.google.protobuf.SingleFieldBuilderV3<
                com.google.devtools.cloudtrace.v2.TruncatableString,
                com.google.devtools.cloudtrace.v2.TruncatableString.Builder,
                com.google.devtools.cloudtrace.v2.TruncatableStringOrBuilder>(
                getBuildId(), getParentForChildren(), isClean());
        buildId_ = null;
      }
      return buildIdBuilder_;
    }
<pre>
A unique identifier for the module, usually a hash of its
contents (up to 128 bytes).
</pre>

<code>.google.devtools.cloudtrace.v2.TruncatableString build_id = 2;</code>
java-trace/proto-google-cloud-trace-v2/src/main/java/com/google/devtools/cloudtrace/v2/Module.java