Due to its remote origin (applets are usually downloaded from a server on the Internet) an applet is a potentially dangerous piece of code. By writing malicious applets, programmers might take control of other people´s machines, copying or deleting data, formatting disks and so on. To minimize these risks, heavy safety restrictions are applicable to java applets executed within a WWW browser. One of these safety restrictions prevents java applets from accessing the user´s local disks (neither in reading nor writing mode) something that, in principle, means that our PV applet cannot save or load local simulation files.

Fortunately, after the Java 1.2 API release users are allowed to grant special permissions to the browser-applet runtime environment. Basically, the new safety plan implemented by Sun Microsystems consists on the following: applets cannot do anything potentially dangerous unless you explicitly allow them to do it.

Very early in the software design process it was clear that the PV simulator applet should allow users to save and load simulation files. This target, however, was difficult to achieve because of the safety restrictions commented above. Three different solutions were found and tested:

1. Convert the PV simulator into a "Signed applet". By using signed certificates, these applets are exposed to lighter safety restrictions so they can read or write on the user´s local disks.
2. Allow all users to save their simulation files on the server the applet was downloaded from.
3. Offer configuration tools so that only users interested in saving and loading local simulation files can do it after configuring their runtime environments.

Of these three options only the third has been implemented in the final version of the program. It has proved the best solution from the server´s organization and security point of view. As regards the user own safety it is also the best solution because the applet will only have permission to access a particular folder in the local machine, while signed applets can read or write anything anywhere.

To summarize, if you want to save or load simulation files on your local machine you must do the following:

• Install a WWW browser compatible with Java 1.2 (or higher). While writing this help page the latest releases of Netscape and Internet Explorer are the versions 6.1 and 5.5 respectively. Netscape 6 with the Java 1.3.0_01 plug-in has been successfully tested. Up-to-date browsers can be found in http://home.netscape.com/download/ and http://www.microsoft.com/download/. If, by any reasons, you want to keep your old browser, it can be updated by installing a Java plug-in (please visit http://www.javasoft.com to find the latest release). Should your browser´s Java version be unknown to you, please click on the following link and it will be detected: JavaInfo
• Find in your computer a file called  java.security. If you have installed any Java programming environments or Sun´s JDKs or JREs you well may find several versions of this file in different paths. In that case, the only relevant file is the one located in the folder lib/security in your java.home path. This folder is created during the installation of any JDK (Java Development Kit), SDK (Software Development Kit) or JRE (Java Runtime Environment). The resulting path to the wanted file might be something like the following: C:\Program Files\JavaSoft\JRE\1.3.0_01\lib\security\java.security
• After finding the right version of the  java.security file, open it with a text editor and check its contents. This file should be similar to the following: default_java.security including, by the middle of the file, these two lines: 

            policy.url.1=file:${java.home}/lib/security/java.policy
            policy.url.2=file:${user.home}/.java.policy

            After these two lines you must include the following:

            policy.url.3=file:${user.home}/fotorred.java.policy

            so that your new java.security file looks like this one: new_java.security

• In your user.home folder (if you are using Windows it is very likely that this path is C:\Windows) create a file called fotorred.java.policy with the following contents:

             grant {
                permission java.util.PropertyPermission "user.home","read";
                permission java.io.FilePermission "${user.home}${/}PVSIM${/}*", "read, write";
            };

This file grants read permission to find your user.home folder and read-write permissions to access files in a folder called PVSIM (in capitals) located in your user.home path. Access to any other file in your computer will not be allowed. Later on, the applet sets even higher safety restrictions checking that any file written to or read from that path has ".pvs" extension (containing PV simulation data only).

• After editing and saving the files java.security and fotorred.java.policy execute the applet in your browser as usual. If all the steps described above have been followed, you should be able to save and load simulation files in the PVSIM folder (you must create this folder yourself).

    If you are having trouble to find your java.home and user.home folders, download and execute this small Java application: user_paths. This program tries to find these folders in your machine and displays their paths. It is not an applet (security restrictions prevent applets from reading this information) so it is not executed in a browser. You should download the program, save it on your disk, unzip it and execute it by double clicking on the file called launch.bat. If your computer is capable of executing Java applications (it will be if you have ever installed a JDK or a JRE) a window will be displayed showing the path of the two wanted folders.

    If your machine cannot execute the application described above and you cannot find your user.home and java.home folders yourself, it looks like your computer is not prepared to execute Java programs and cannot grant security permissions. To learn more about how to configure your machine to work with Java please visit http://www.javasoft.com.