• POIFS client- wants to read content of file system
• POIFS - understands POIFS file system

1. POIFS client requests POIFS to read a POIFS file system, providing an InputStream containing POIFS file system in question.
2. POIFS reads from the InputStream in 512 byte blocks
3. POIFS verifies that the first block begins with the well known signature (0xE11AB1A1E011CFD0)
4. POIFS reads the Block Allocation Table from the first block and, if necessary, from the XBAT blocks.
5. POIFS obtains the start block of the Property Table and reads the Property Table (use case 9, read file)
6. POIFS reads the individual entries in the Property Table
7. POIFS obtains the start block of the Small Block Allocation Table and reads the Small Block Allocation Table (use case 9, read file)
8. POIFS obtains the start block of the Small Block store from the first entry in the Property Table and reads the Small Block Array (use case 9, read file)

• 2a. If the last block read is not a 512 byte block, the InputStream is not that of a POIFS file system, and POIFS throws an appropriate exception.
• 3a. If the signature is incorrect, the InputStream is not that of a POIFS file system, and POIFS throws an appropriate exception.

• POIFS client- wants to write file system out.
• POIFS - knows how to write file system out.

• File system has been read (use case 1, read existing file system) and subsequently modified (use case 4, replace file in file system; use case 5, delete file from file system; or use case 6, write new file to file system; in any combination)

or

• File system has been created (use case 3, create new file system)

1. POIFS client provides an OutputStream to write the file system to.
2. POIFS gets the sizes of the Property Table and each file in the file system.
3. If any files in the file system requires storage in a Small Block Array, POIFS creates a Small Block Array of sufficient size to hold all of the small files.
4. POIFS calculates the number of big blocks needed to hold all of the large files, the Property Table, and, if necessary, the Small Block Array and the Small Block Allocation Table.
5. POIFS creates a set of big blocks sufficient to store the Block Allocation Table
6. POIFS creates and writes the header block
7. POIFS writes out the XBAT blocks, if needed.
8. POIFS writes out the Small Block Array, if needed
9. POIFS writes out the Small Block Allocation Table, if needed
10. POIFS writes out the Property Table
11. POIFS writes out the large files, if needed
12. POIFS closes the OutputStream.

• 6a. Exceptions writing to the OutputStream will be propagated back to the POIFS client.
• 7a. Exceptions writing to the OutputStream will be propagated back to the POIFS client.
• 8a. Exceptions writing to the OutputStream will be propagated back to the POIFS client.
• 9a. Exceptions writing to the OutputStream will be propagated back to the POIFS client.
• 10a. Exceptions writing to the OutputStream will be propagated back to the POIFS client.
• 11a. Exceptions writing to the OutputStream will be propagated back to the POIFS client.
• 12a. Exceptions closing the OutputStream will be propagated back to the POIFS client.

• POIFS client- wants to create a new file system
• POIFS - knows how to create a new file system

1. POIFS creates an empty Property Table.

• POIFS client- wants to replace an existing file in the file system
• POIFS - knows how to manage the file system

Either

• The file system has been read (use case 1, read existing file system) and a file has been extracted from the file system (use case 7, read existing file from file system)

or

• The file system has been created (use case 3, create new file system) and a file has been written to the file system (use case 6, write new file to file system)

1. POIFS discards storage of the existing file.
2. POIFS updates the existing file's entry in the Property Table
3. POIFS stores the new file's data

• 1a. POIFS throws an exception if the file does not exist.

• POIFS client- wants to remove a file from a file system
• POIFS - knows how to manage the file system

Either

• The file system has been read (use case 1, read existing file system) and a file has been extracted from the file system (use case 7, read existing file from file system)

or

• The file system has been created (use case 3, create new file system) and a file has been written to the file system (use case 6, write new file to file system)

1. POIFS discards the specified file's storage.
2. POIFS discards the file's Property Table entry.

• 1a. POIFS throws an exception if the file does not exist.

• POIFS client- wants to add a new file to the file system
• POIFS - knows how to manage the file system

1. The POIFS client provides a file name
2. POIFS creates a new Property Table entry for the new file
3. POIFS provides the POIFS client with an OutputStream to write to.
4. The POIFS client writes data to the provided OutputStream.
5. The POIFS client closes the provided OutputStream
6. POIFS updates the Property Table entry with the new file's size

• 1a. POIFS throws an exception if a file with the specified name already exists in the file system.
• 1b. POIFS throws an exception if the file name is too long. The limit on file name length is 31 characters.

• POIFS client- wants to read a file from the file system
• POIFS - knows how to manage the file system

• The file system has been read (use case 1, read existing file system) or has been created and written to (use case 3, create new file system; use case 6, write new file to file system).
• The specified file exists in the file system.

1. The POIFS client provides the name of a file to be read
2. POIFS provides an InputStream to read from.
3. The POIFS client reads from the InputStream.
4. The POIFS client closes the InputStream.

• POIFS client- wants to know what files exist in the file system
• POIFS - knows how to manage the file system

1. The POIFS client requests the file system directory.
2. POIFS returns an Iterator. The Iterator will not include the root entry in the Property Table, and may be an Iterator over an empty Collection.

• POIFS - POIFS needs to read a file, or something resembling a file (i.e., the Property Table, the Small Block Array, or the Small Block Allocation Table)

1. POIFS begins with a start block, a file size, and a flag indicating whether to use the Big Block Allocation Table or the Small Block Allocation Table
2. POIFS returns an InputStream.
3. Reads from the InputStream are performed by walking the specified Block Allocation Table and reading the blocks indicated.
4. POIFS closes the InputStream when finished reading the file, or its client wants to close the InputStream.

• POIFS client- wants to rename an existing file in the file system.
• POIFS - knows how to manage the file system.

• The file system is has been read (use case 1, read existing file system) or has been created and written to (use case 3, create new file system; use case 6, write new file to file system.
• The specified file exists in the file system.
• The new name for the file does not duplicate another file in the file system.

1. POIFS updates the Property Table entry for the specified file with its new name.

• 1a. If the old file name is not in the file system, POIFS throws an exception.
• 1b. If the new file name already exists in the file system, POIFS throws an exception.
• 1c. If the new file name is too long (the limit is 31 characters), POIFS throws an exception.