publicId 和 systemId 究竟是个什么东东？

org.xml.sax.InputSource中有publicId 和 systemId
这个public identifier 和 system identifier究竟是个啥玩意儿？
谁跟我讲讲啊？
[code="java"]

/**
 * Set the public identifier for this input source.
 *
 * <p>The public identifier is always optional: if the application
 * writer includes one, it will be provided as part of the
 * location information.</p>
 *
 * @param publicId The public identifier as a string.
 * @see #getPublicId
 * @see org.xml.sax.Locator#getPublicId
 * @see org.xml.sax.SAXParseException#getPublicId
 */
public void setPublicId (String publicId)
{
this.publicId = publicId;
}

/**
 * Set the system identifier for this input source.
 *
 * <p>The system identifier is optional if there is a byte stream
 * or a character stream, but it is still useful to provide one,
 * since the application can use it to resolve relative URIs
 * and can include it in error messages and warnings (the parser
 * will attempt to open a connection to the URI only if
 * there is no byte stream or character stream specified).</p>
 *
 * <p>If the application knows the character encoding of the
 * object pointed to by the system identifier, it can register
 * the encoding using the setEncoding method.</p>
 *
 * <p>If the system identifier is a URL, it must be fully
 * resolved (it may not be a relative URL).</p>
 *
 * @param systemId The system identifier as a string.
 * @see #setEncoding
 * @see #getSystemId
 * @see org.xml.sax.Locator#getSystemId
 * @see org.xml.sax.SAXParseException#getSystemId
 */
public void setSystemId (String systemId)
{
this.systemId = systemId;
}

[/code]

看看Wiki上的解释吧。
[quote]
A public identifier is a document processing construct in SGML and XML.

In HTML and XML, a public identifier is meant to be universally unique within its application scope. It typically occurs in a Document Type Declaration.

A public identifier is meant to identify a document type that may span more than one application.
[/quote]
[quote]
A system identifier is meant for a document type that is used exclusively in one application.
[/quote]