The Parser interface
The org.apache.tika.parser.Parser interface is the key concept of Apache Tika. It hides the complexity of different file formats and parsing libraries while providing a simple and powerful mechanism for client applications to extract structured text content and metadata from all sorts of documents. All this is achieved with a single method:
void parse(InputStream stream, ContentHandler handler, Metadata metadata)
throws IOException, SAXException, TikaException;The parse method takes the document to be parsed and related metadata as input and outputs the results as XHTML SAX events and extra metadata. The main criteria that lead to this design were:
- Streamed parsing
- The interface should require neither the client application nor the parser implementation to keep the full document content in memory or spooled to disk. This allows even huge documents to be parsed without excessive resource requirements.
- Structured content
- A parser implementation should be able to include structural information (headings, links, etc.) in the extracted content. A client application can use this information for example to better judge the relevance of different parts of the parsed document.
- Input metadata
- A client application should be able to include metadata like the file name or declared content type with the document to be parsed. The parser implementation can use this information to better guide the parsing process.
- Output metadata
- A parser implementation should be able to return document metadata in addition to document content. Many document formats contain metadata like the name of the author that may be useful to client applications.
These criteria are reflected in the arguments of the parse method.
Document input stream
The first argument is an InputStream for reading the document to be parsed.
If this document stream can not be read, then parsing stops and the thrown IOException is passed up to the client application. If the stream can be read but not parsed (for example if the document is corrupted), then the parser throws a TikaException.
The parser implementation will consume this stream but will not close it. Closing the stream is the responsibility of the client application that opened it in the first place. The recommended pattern for using streams with the parse method is:
InputStream stream = ...; // open the stream
try {
parser.parse(stream, ...); // parse the stream
} finally {
stream.close(); // close the stream
}Some document formats like the OLE2 Compound Document Format used by Microsoft Office are best parsed as random access files. In such cases the content of the input stream is automatically spooled to a temporary file that gets removed once parsed. A future version of Tika may make it possible to avoid this extra file if the input document is already a file in the local file system. See TIKA-153 for the status of this feature request.
XHTML SAX events
The parsed content of the document stream is returned to the client application as a sequence of XHTML SAX events. XHTML is used to express structured content of the document and SAX events enable streamed processing. Note that the XHTML format is used here only to convey structural information, not to render the documents for browsing!
The XHTML SAX events produced by the parser implementation are sent to a ContentHandler instance given to the parse method. If this the content handler fails to process an event, then parsing stops and the thrown SAXException is passed up to the client application.
The overall structure of the generated event stream is (with indenting added for clarity):
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>...</title>
</head>
<body>
...
</body>
</html>Parser implementations typically use the XHTMLContentHandler utility class to generate the XHTML output.
Dealing with the raw SAX events can be a bit complex, so Apache Tika (since version 0.2) comes with a number of utility classes that can be used to process and convert the event stream to other representations.
For example, the BodyContentHandler class can be used to extract just the body part of the XHTML output and feed it either as SAX events to another content handler or as characters to an output stream, a writer, or simply a string. The following code snippet parses a document from the standard input stream and outputs the extracted text content to standard output:
ContentHandler handler = new BodyContentHandler(System.out); parser.parse(System.in, handler, ...);
Another useful class is ParsingReader that uses a background thread to parse the document and returns the extracted text content as a character stream:
InputStream stream = ...; // the document to be parsed
Reader reader = new ParsingReader(parser, stream, ...);
try {
...; // read the document text using the reader
} finally {
reader.close(); // the document stream is closed automatically
}Document metadata
The final argument to the parse method is used to pass document metadata both in and out of the parser. Document metadata is expressed as an Metadata object.
The following are some of the more interesting metadata properties:
- Metadata.RESOURCE_NAME_KEY
- The name of the file or resource that contains the document.
A client application can set this property to allow the parser to use file name heuristics to determine the format of the document.
The parser implementation may set this property if the file format contains the canonical name of the file (for example the Gzip format has a slot for the file name).
- Metadata.CONTENT_TYPE
- The declared content type of the document.
A client application can set this property based on for example a HTTP Content-Type header. The declared content type may help the parser to correctly interpret the document.
The parser implementation sets this property to the content type according to which the document was parsed.
- Metadata.TITLE
- The title of the document.
The parser implementation sets this property if the document format contains an explicit title field.
- Metadata.AUTHOR
- The name of the author of the document.
The parser implementation sets this property if the document format contains an explicit author field.
Note that metadata handling is still being discussed by the Tika development team, and it is likely that there will be some (backwards incompatible) changes in metadata handling before Tika 1.0.
Parser implementations
Apache Tika comes with a number of parser classes for parsing various document formats. You can also extend Tika with your own parsers, and of course any contributions to Tika are warmly welcome.
The goal of Tika is to reuse existing parser libraries like PDFBox or Apache POI as much as possible, and so most of the parser classes in Tika are adapters to such external libraries.
Tika also contains some general purpose parser implementations that are not targeted at any specific document formats. The most notable of these is the AutoDetectParser class that encapsulates all Tika functionality into a single parser that can handle any types of documents. This parser will automatically determine the type of the incoming document based on various heuristics and will then parse the document accordingly.
