如何使用命令按钮而不是上传按钮上传p：fileUpload的文件

Iam using JSF2.1 along with primefaces for uploading file.In my application i have to add the file dynamically on creating a record.But when i use i cant write the code for uploading my file during save.I want the file to be uploaded on click of save only and not during upload.Can anyone help me how to implement thisIam using JSF2.1 along with primefaces for uplo