Groovy Script setLength Error with StringBuilder and How to Work Around It

While writing a Groovy script I encountered a strange error: calling sb.setLength(0) on a StringBuilder caused a No signature of method: java.lang.StringBuilder.setLength() error.

Groovy automatically creates a property for getter/setter pairs, so a method named getFun() / setFun() would generate a fun property, and vice‑versa; this flexibility can hide missing methods.

Original code that triggered the error:

StringBuilder sb = new StringBuilder()<br/>for (int i = 0; i < 3; i++) {<br/>    sb.append("a" + i)<br/>    output(sb.toString())<br/>    sb.setLength(0)<br/>}<br/>

The Java AbstractStringBuilder#setLength(int newLength) method either truncates the string when newLength is smaller than the current length or pads it with null characters when larger.

The exact error message was:

No signature of method: java.lang.StringBuilder.setLength() is applicable for argument types: (Integer) values: [0]<br/>    at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.unwrap(ScriptBytecodeAdapter.java:70)<br/>    ...

Because Groovy treats StringBuilder differently, the call was resolved as a property access rather than a method call, and the integer argument was seen as an Integer instead of the expected int, leading to the failure.

To test the hypothesis, I added a meta‑class property:

StringBuilder.metaClass.length = 32<br/>StringBuilder sb = new StringBuilder()<br/>for (int i = 0; i < 3; i++) {<br/>    sb.append("a" + i)<br/>    output(sb.toString())<br/>    sb.setLength(0)<br/>}<br/>

This removed the error but also disabled the actual setLength functionality because the property took precedence over the method.

Finally, I removed the setLength call entirely:

StringBuilder.metaClass.length = 32<br/>StringBuilder sb = new StringBuilder()<br/>for (int i = 0; i < 3; i++) {<br/>    sb.append("a" + i)<br/>    output(sb.toString())<br/>}<br/>

That solved the problem in the current project, but the issue could not be reproduced in another Maven/Gradle project. Updating the Groovy dependency and the compilation plugin version eliminated the bug, suggesting it was a version‑specific Groovy compilation quirk.