struts2多文件上传问题

异常：
java.lang.NullPointerException
com.eileen.upload.UploadMoreAction.execute(UploadMoreAction.java:49)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
java.lang.reflect.Method.invoke(Method.java:597)
com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:441)
com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:280)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:243)
com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:165)
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)
我的Action代码：
public String execute() throws Exception {
InputStream input = null;
OutputStream output = null;
if(uploads != null){
for(int i = 0;i < uploads.size();i++){
input = new FileInputStream(uploads.get(i)); //创建一个输入流对象
output = new FileOutputStream("g:\\upload\\" + uploadFileNames.get(i)); //创建一个输出流对象 byte buffer[] = new byte[8192]; //创建一个字节数组
int count = 0;
while((count = input.read(buffer)) > 0){
output.write(buffer, 0, count); //将buffer数组中从0开始的count个字节写入此输出流
}
output.close(); //关闭输出流
input.close(); //关闭输入流
}

}
return Action.SUCCESS;
}