XML is a way to organize information on the internet. It's like a recipe that tells computers how to understand and use the information. Entities in XML are like ingredients in a recipe. They help you define and reuse parts of your information, making it easier to manage and share.
What is XML?
XML stands for Extensible Markup Language. XML is like a language that computers use to understand and organize information. It's like a set of instructions that tells the computer how to read and process the information. XML uses tags to label different parts of the information, making it easy for both humans and computers to understand.
What is the entity in XML?
An entity in XML is a way to store and reuse information, like a bookmark or a sticky note. It's a placeholder that stands for a piece of text or data. For example, instead of writing a company name many times in an XML document, you can create an entity for the company name and use it wherever needed.
What is an XML Entity?
An entity in XML is like a placeholder for text or data. It's a way to store information that you can reuse in your XML document. For example, instead of writing the full name of a company multiple times in your document, you can use an entity to represent that name and then use the entity wherever you need the name to appear.
Use of Entities in XML Document
Entities in XML are like magic words that help you organize your document and make it easier to work with. Entities in XML are like nicknames for long words or phrases. Instead of writing out the long word every time, you can use the nickname to save time and space. This makes your document easier to read and edit.
Defining standard phrases or company names that are used frequently in the document. Including content from external sources, such as text or data files, into the XML document. Representing special characters that have special meaning in XML, such as <, >, and &.
Types of Entities
There are generally three types of entities in XML
- Internal entities.
- External entities.
- Parametric entities.
What is Internal entities
Internal entities are like sticky notes that you write and keep within a book. They are bits of information that you define inside your XML document and can use throughout the document wherever you need them. They are useful for defining short, reusable pieces of text. For example, you can define an internal entity for a company name and then use that entity wherever you need to display the company name in your XML document.
To create an internal entity, you first need to define it in the document type definition (DTD) section of your XML document. This is where you tell the XML parser that you're creating an entity and what its value should be.
Once you've defined an internal entity, you can use it anywhere in your XML document by referencing its name with an ampersand (&) at the beginning and a semicolon (;) at the end. When the XML document is processed, the parser replaces the entity reference with the actual value of the entity.
Declaration of an Entity
Declaring an entity in XML is like giving a name to a piece of text or data so you can use it later. To declare an entity, you use a special syntax in the document type definition (DTD) section of your XML document.
Here is a simple example:
<!DOCTYPE note [
<!ENTITY company "Example Corp">
]>
In this example, we're declaring an entity called "company" with the value "Example Corp". This means that whenever we use "&company;" in our XML document, it will be replaced with "Example Corp".
You can declare entities for various purposes, like storing company names, addresses, or any other text or data that you want to reuse in your document.
Declaration of an Internal Entity
Declaring an internal entity in XML is like giving a nickname to a piece of text or data inside your document. It's a way to define a short, reusable name for something longer.
Here is how we do it:
- In your XML document, you write '<!DOCTYPE' to start the declaration.
- Then you write the name you want to use for your entity, like 'company'.
- After that, you write the value you want your entity to represent, like "Example Corp".
- Finally, you close the declaration with '>'.
For example, if you want to declare an entity for the company name "Example Corp", you would write:
<!DOCTYPE note [
<!ENTITY company "Example Corp">
]>
After declaring the entity, you can use '&company;' in your XML document wherever you want to use the company name. When the XML document is processed, '&company;' will be replaced with "Example Corp".
Predefined Internal Entities
Predefined internal entities are special entities in XML that are already defined and can be used without needing to be declared. These entities are used to represent characters that have special meaning in XML, such as `<`, `>`, `&`, `"`, and `'`.
Here are the predefined internal entities in XML:
- `<` represents `<`
- `>` represents `>`
- `&` represents `&`
- `"` represents `"`
- - `'` represents `'`
For example, if you want to include the `<` character in your XML document as text (rather than as part of a tag), you would use `<`. Similarly, if you want to include an ampersand (`&`) as text, you would use `&`.
Using predefined internal entities helps ensure that your XML document is well-formed and can be properly processed by XML parsers.
What are External entities
External entities are like bookmarks that point to information outside of your XML document. Instead of storing the information directly in your XML file, you can use an external entity to refer to the information stored in another file. This keeps your XML document neat and tidy. This is useful for including large amounts of text or data that you don't want to include directly in the XML document.
To use an external entity, you first need to define it in the document type definition (DTD) section of your XML document. In the DTD, you specify the name of the entity and the location of the external file that contains the content you want to include.
Here is an example:
<!DOCTYPE note [
<!ENTITY externalEntity SYSTEM "external_file.xml">
]>
In this example, we're defining an external entity called 'externalEntity' that points to the file "external_file.xml". This file could contain any kind of content, such as text or data, that you want to include in your XML document.
Once you have defined an external entity, you can use it in your XML document like this:
<note>
<content>&externalEntity;</content>
</note>
When the XML document is processed, the content of "external_file.xml" will be inserted in place of '&externalEntity;' , effectively including the external content in your XML document.
External entities are useful for including large amounts of content or for including content that is shared among multiple XML documents. They allow you to keep your XML documents organized and make them easier to maintain.
Declaration of External Entity
Declaring an external entity in XML is like telling the XML parser where to find the information you want to include in your document. It's a way to create a placeholder for content that is stored in an external file.
Here is how you declare an external entity:
- Start with '<!DOCTYPE' to indicate that you're defining a document type.
- Next, specify the name you want to use for your entity, like 'externalEntity'.
- Then, use the keyword 'SYSTEM' to indicate that the entity is external.
- Finally, provide the file path or URL where the external content is located, like "external_file.xml".
Here is an example:
<!DOCTYPE note [
<!ENTITY externalEntity SYSTEM "external_file.xml">
]>
In this example, we're declaring an external entity called 'externalEntity' that points to the file "external_file.xml". This file contains the content that we want to include in our XML document.
After declaring the external entity, you can use it in your XML document just like an internal entity. When the XML document is processed, the content of the external file will be inserted in place of the entity reference.
Parametric Entity
Parameter entities in XML are like variables that you can use to define parts of your document type definition (DTD) in XML.
To define a parameter entity, you use the % symbol followed by the entity name and its value.
Here is an example:
<!ENTITY % myEntity "some value">
You can then use the parameter entity in your DTD wherever you would use the value it represents. Here is an example:
<!ELEMENT myElement (%myEntity;)>
When the DTD is processed, the parameter entity '%myEntity;' will be replaced with its value, "some value", in this case. This allows you to define parts of your DTD in one place and reuse them throughout the DTD, making your DTD more concise and easier to maintain.
Parameter entities are particularly useful when you have parts of your DTD that are used in multiple places or change easily. By defining these parts as parameter entities, you can make your DTD more flexible and adaptable to different needs.
Conclusion
In conclusion, XML entities are a powerful feature that allow you to define reusable pieces of text or include content from other sources in your XML document. By understanding how entities work and how to use them effectively, you can make your XML documents more flexible and powerful.