iTextSharp：.NET开发者的PDF处理指南

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《iTextSharp在.NET框架中的PDF处理指南》随着.NET框架的局限性，对于在ASP.NET Web应用程序中处理PDF文件的需求，开发者通常会寻求第三方组件的支持。iTextSharp作为一款知名的Java工具iText的.NET版本，提供了强大的PDF处理能力。本文档旨在介绍如何在.NET环境下利用iTextSharp进行PDF操作，包括基础的PDF语法、对象系统以及文件结构。 **第1章：PDF对象系统** - **1.1 基本PDF对象**：PDF文档由一系列的对象组成，如字节流、文本、图像等。理解这些基本对象类型是操作PDF文件的基础。 - **1.2 iText's PdfObject实现**：iTextSharp库封装了PDF对象，使得.NET开发者能够更方便地操作，如PdfReader用于读取PDF，PdfStamper用于添加注释或水印，PdfWriter用于创建新PDF等。 - **1.3 直接对象与间接对象的区别**：直接对象（Direct Objects）是当前页的引用，而间接对象（Indirect Objects）是跨页引用，有助于节省存储空间。了解这种区别有助于高效管理和操作PDF文档。 - **1.4 总结**：本章概述了PDF对象系统的核心概念，为后续操作奠定了基础。 **第2章：PDF文件结构** - **2.1 PDF内部结构**：PDF文件由多个部分组成，如文档信息、页面树、对象流、跨页链接等。理解文件结构有助于开发者正确解析和创建PDF文档。 - **2.2 页面和资源管理**：PDF由多个页面组成，每个页面有自己的资源，如字体、图像等。掌握如何管理这些资源对于生成专业级PDF至关重要。通过阅读这本书，《The ABC of ITextPdf》为.NET开发者提供了一个全面的iTextSharp教程，从入门到深入，涵盖了创建、编辑、以及处理PDF文件的各个方面。它不仅适合初学者学习基本操作，也适合有一定经验的开发人员优化他们的PDF处理代码。由于该书是基于Leanpub平台的动态出版物，作者鼓励读者提供反馈并参与迭代过程，以确保内容的实用性和与时俱进。如果你正在寻找一个实用且灵活的PDF处理解决方案，iTextSharp在.NET框架中的应用是一个值得探索的方向。通过学习本书，你可以更好地理解和利用iTextSharp的强大功能，提升你的ASP.NET应用程序的PDF交互体验。

PDF Objects 10

The elements of the array are stored in an ArrayList. The toString() method of the PdfArray

class returns the toString() output of this ArrayList: the values of the separate objects delimited

with a comma and enclosed by square brackets. The getBytes() method returns null.

You can ask a PdfArray for its size, and use this size to get the different elements of the array one by one. In

this case, we use the getPdfObject() method. We’ll discover some more methods to retrieve elements from

an array in section 1.3.

com.itextpdf.text.pdf.PdfArray:

-> array? true

-> type: 5

-> toString: [/First, Second, 3, false]

-> size: 4

-> Values: /First Second 3 false

com.itextpdf.text.pdf.PdfRectangle:

-> array? true

-> type: 5

-> toString: [0, 0, 595, 842]

-> size: 4

-> Values: 0 0 595 842

In our example, we created a PdfRectangle using only two values 595 and 842. However, a rectangle needs

four values: two for the coordinate of the lower-left corner, two for the coordinate of the upper-right corner.

As you can see, iText added two zeros for the coordinate of the lower-left coordinate.

1.2.6 PdfDictionary

There are only two constructors for the PdfDictionary class. With the empty constructor, you can create

an empty dictionary, and then add entries using the put() method. The constructor that accepts a PdfName

object will create a dictionary with a /Type entry and use the name passed as a parameter as its value. This

entry identifies the type of object the dictionary describes. In some cases, a /SubType entry is used to further

identify a specialized subcategory of the general type.

In code sample 1.6, we create a custom dictionary and an action.

Code sample 1.6: C0106_DictionaryObject

1 public static void main(String[] args) {

2 PdfDictionary dict = new PdfDictionary(new PdfName("Custom"));

3 dict.put(new PdfName("Entry1"), PdfName.FIRST);

4 dict.put(new PdfName("Entry2"), new PdfString("Second"));

5 dict.put(new PdfName("3rd"), new PdfNumber(3));

6 dict.put(new PdfName("Fourth"), PdfBoolean.PDFFALSE);

7 showObject(dict);

8 showObject(PdfAction.gotoRemotePage("test.pdf", "dest", false, true));

9 }

PDF Objects 12

As explained in table 1.1, a PDF dictionary is stored as a series of key value pairs enclosed by << and >>. The

action created in code sample 1.6 looks like this when viewed in a plain text editor:

<</D(dest)/F(test.pdf)/S/GoToR/NewWindow true>>

The basic PdfDictionary object has plenty of subclasses such as PdfAction, PdfAnnotation, PdfCollection,

PdfGState, PdfLayer, PdfOutline, etc. All these subclasses serve a specific purpose and they were created

to make it easier for developers to create objects without having to worry too much about the underlying

structures.

1.2.7 PdfStream

The PdfStream class also extends the PdfDictionary object. A stream object always starts with a dictionary

object that contains at least a /Length entry of which the value corresponds with the number of stream bytes.

For now, we’ll only use the constructor that accepts a byte[] as parameter. The other constructor involves

a PdfWriter instance, which is an object we haven’t discussed yet. Although that constructor is mainly for

internal use —it offers an efficient, memory friendly way to write byte streams of unknown length to a PDF

document—, we’ll briefly cover this alternative constructor in the Create your PDFs with iText² tutorial.

Code sample 1.7: C0107_StreamObject

1 public static void main(String[] args) {

2 PdfStream stream = new PdfStream(

3 "Long stream of data stored in a FlateDecode compressed stream object"

4 .getBytes());

5 stream.flateCompress();

6 showObject(stream);

7 }

8 public static void showObject(PdfStream obj) {

9 System.out.println(obj.getClass().getName() + ":");

10 System.out.println("-> stream? " + obj.isStream());

11 System.out.println("-> type: " + obj.type());

12 System.out.println("-> toString: " + obj.toString());

13 System.out.println("-> raw length: " + obj.getRawLength());

14 System.out.println("-> size: " + obj.size());

15 for (PdfName key : obj.getKeys()) {

16 System.out.print(" " + key + ": ");

17 System.out.println(obj.get(key));

18 }

19 }

In the lines following the fully qualified name, we see that the isStream() method returns true and the

type() method returns 7. The toString() method returns nothing more than the word "Stream".

²https://leanpub.com/itext_pdfcreate

PDF Objects 13

We can store the long String we used in code sample 1.7 “as is” inside the stream. In this case,

invoking the getBytes() method will return the bytes you used in the constructor.

If a stream is compressed, for instance by using the flateCompress() method, the getBytes()

method will return null. In this case, the bytes are stored inside a ByteArrayOutputStream and you

can write these bytes to an OutputStream using the writeContent() method. We didn’t do that

because it doesn’t make much sense for humans to read a compressed stream.

The PdfStream instance remembers the original length aka the raw length. The length of the compressed

stream is stored in the dictionary.

com.itextpdf.text.pdf.PdfStream:

-> stream? true

-> type: 7

-> toString: Stream

-> raw length: 68

-> size: 2

/Filter: /FlateDecode

/Length: 67

In this case, compression didn’t make much sense: 68 bytes were compressed into 67 bytes. In theory, you could

choose a different compression level. The PdfStream class has different constants such as NO_COMPRESSION (0),

BEST_SPEED (1) and BEST_COMPRESSION (9). In practice, we’ll always use DEFAULT_COMPRESSION (-1).

1.2.8 PdfNull

We’re using the PdfNull class internally in some very specific cases, but there’s very little chance you’ll ever

need to use this class in your own code. For instance: it’s better to remove an entry from a dictionary than to

set its value to null; it saves the PDF consumer processing time when parsing the files you’ve created.

Code sample 1.8: C0108_NullObject

1 public static void main(String[] args) {

2 showObject(PdfNull.PDFNULL);

3 }

4 public static void showObject(PdfNull obj) {

5 System.out.println(obj.getClass().getName() + ":");

6 System.out.println("-> type: " + obj.type());

7 System.out.println("-> bytes: " + new String(obj.getBytes()));

8 System.out.println("-> toString: " + obj.toString());

9 }

The output of code sample 1.8 is pretty straight-forward: the fully qualified name of the class, its type (8) and

the output of the getBytes() and toString() methods.

PDF Objects 14

com.itextpdf.text.pdf.PdfNull:

-> type: 8

-> bytes: null

-> toString: null

These were the eight basic types, numbered from 1 to 8. Two more numbers are reserved for specific PdfObject

classes: 0 and 10. Let’s start with the class that returns 0 when you call the type() method.

1.2.9 PdfLiteral

The objects we’ve discussed so far were literally the first objects that were written when I started writing

iText. Since 2000, they’ve been used to build billions of PDF documents. They form the foundation of iText’s

object-oriented approach to create PDF documents.

Working in an object-oriented way is best practice and it’s great, but for some straight-forward objects, you

wish you’d have a short-cut. That’s why we created PdfLiteral. It’s an iText object you won’t find in the PDF

specification or ISO-32000-1 or -2. It allows you to create any type of object with a minimum of overhead.

For instance: we often need an array that defines a specific matrix, called the identity matrix. It consists of

six elements: 1, 0, 0, 1, 0 and 0. Should we really create a PdfArray object and add these objects one by one?

Wouldn’t it be easier if we just created the literal array: [1 0 0 1 0 0]?

That’s what PdfLiteral is about. You create the object passing a String or a byte[]; you can even pass the

object type to the constructor.

Code sample 1.9: C0109_LiteralObject

1 public static void main(String[] args) {

2 showObject(PdfFormXObject.MATRIX);

3 showObject(new PdfLiteral(

4 PdfObject.DICTIONARY, "<</Type/Custom/Contents [1 2 3]>>"));

5 }

6 public static void showObject(PdfObject obj) {

7 System.out.println(obj.getClass().getName() + ":");

8 System.out.println("-> type: " + obj.type());

9 System.out.println("-> bytes: " + new String(obj.getBytes()));

10 System.out.println("-> toString: " + obj.toString());

11 }

The MATRIX constant used in code sample 1.9 was created like this: new PdfLiteral("[1 0 0 1 0 0]"); when

we write this object to a PDF, it is treated in exactly the same way as if we’d had created a PdfArray, except

that its type is 0 because PdfLiteral doesn’t parse the String to check the type.

We also create a custom dictionary, telling the object its type is PdfObject.DICTIONARY. This doesn’t have

any impact on the fully qualified name. As the String passed to the constructor isn’t being parsed, you can’t

ask the dictionary for its size nor get the key set of the entries.

The content is stored literally, as indicated in the name of the class: PdfLiteral.

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