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PDF files can be protected by two passwords: a user password and an owner password. If a PDF is protected with a user password, you ll have to enter this password before you can open the document in Adobe Reader. If a document has an owner password, you must provide the password along with the constructor when creating a PdfReader
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Accessing an existing PDF with PdfReader
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instance, or a BadPasswordException will be thrown. More details about the different ways you can encrypt a PDF document, and about the different permissions you can set, will follow in chapter 12.
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Reducing the memory use of PdfReader
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In most of this book s examples, you ll create an instance of PdfReader using a String representing the path to the existing PDF file. Using this constructor will cause PdfReader to load plenty of PDF objects (from the file) into Java objects (in memory). This can be overkill for large documents, especially if you re only interested in part of the document. If that s the case, you can choose to read the PDF only partially.
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PARTIAL READS
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Suppose you have a document with 1000 pages. PdfReader will do a full read of these pages, even if you re only interested in page 1. You can avoid this by using another constructor. You can compare the memory used by different PdfReader instances created to read the timetable PDF from chapter 3:
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Listing 6.2 MemoryInfo.java
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public static void main(String[] args) throws IOException { MovieTemplates.main(args); PrintWriter writer = new PrintWriter(new FileOutputStream(RESULT)); fullRead(writer, MovieTemplates.RESULT); partialRead(writer, MovieTemplates.RESULT); writer.close(); } public static void fullRead(PrintWriter writer, String filename) throws IOException { long before = getMemoryUse(); PdfReader reader = new PdfReader(filename); reader.getNumberOfPages(); writer.println(String.format("Memory used by full read: %d", getMemoryUse() - before)); writer.flush(); } public static void partialRead(PrintWriter writer, String filename) throws IOException { long before = getMemoryUse(); PdfReader reader = new PdfReader( new RandomAccessFileOrArray(filename), null); reader.getNumberOfPages(); writer.println(String.format("Memory used by partial read: %d", getMemoryUse() - before)); writer.flush(); }
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The file size of the timetable document from chapter 3 is 15 KB. The memory used by a full read is about 35 KB, but a partial read needs only 4 KB. This is a significant difference. When reading a file partially, more memory will be used as soon as you start working with the reader object, but PdfReader won t cache unnecessary objects. That
Working with existing PDFs
also makes a huge difference, so if you re dealing with large documents, consider using PdfReader with a RandomAccessFileOrArray parameter constructed with a path to a file.
NOTE In part 4, you ll see how to manipulate a PDF at the lowest level. You ll change PDF objects in PdfReader and then save the altered PDF. For this to work, the modified objects need to be cached. Depending on the changes you want to apply, using a PdfReader instance created with a RandomAccessFileOrArray may not be an option.
Another way to reduce the memory usage of PdfReader up front is to reduce the number of pages before you start working with it.
SELECTING PAGES
Next, you ll read the timetable from example 3 once again, but you ll immediately tell PdfReader that you re only interested in pages 4 to 8.
Listing 6.3 SelectPages.java
PdfReader reader = new PdfReader(MovieTemplates.RESULT); reader.selectPages("4-8");
The general syntax for the range that s used in the selectPages() method looks like this:
[!][o][odd][e][even]start[-end]
You can have multiple ranges separated by commas, and the ! modifier removes pages from what is already selected. The range changes are incremental; numbers are added or deleted as the range appears. The start or the end can be omitted; if you omit both, you need at least o (odd; selects all odd pages) or e (even; selects all even pages). If you ask the reader object for the number of pages before selectPages() in listing 6.3, it will tell you that the document has 8 pages. If you do the same after making the page selection, it will tell you that there are only 5 pages: pages 4, 5, 6, 7, and 8. The old page 4 will be the new page 1. Be careful not to try getting information about pages that are outside the new range. Don t add the following line to listing 6.3:
reader.getPageSize(6);
This line will throw a NullPointerException because there are no longer 6 pages in the reader object. Now that you ve had a short introduction to PdfReader, you re ready to start manipulating existing PDF documents.
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