in commons-fileupload2-core/src/main/java/org/apache/commons/fileupload2/core/MultipartInput.java [918:924]
public void setBoundary(final byte[] boundary) throws FileUploadBoundaryException {
if (boundary.length != boundaryLength - BOUNDARY_PREFIX.length) {
throw new FileUploadBoundaryException("The length of a boundary token cannot be changed");
}
System.arraycopy(boundary, 0, this.boundary, BOUNDARY_PREFIX.length, boundary.length);
computeBoundaryTable();
}