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 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
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package freemarker.cache;

import java.io.IOException;
import java.io.Reader;

import freemarker.template.Configuration;
import freemarker.template.TemplateNotFoundException;

FreeMarker loads template "files" through objects that implement this interface, thus the templates need not be real files, and can come from any kind of data source (like classpath, servlet context, database, etc). While FreeMarker provides a few TemplateLoader implementations out-of-the-box, it's normal for embedding frameworks to use their own implementations. 
To set the TemplateLoader used by FreeMaker, use Configuration.setTemplateLoader(TemplateLoader). 
Implementations of this interface should be thread-safe.
Implementations should override Object.toString() to show information about from where the TemplateLoader loads the templates. For example, for a template loader that loads template from database table toString could return something like "MyDatabaseTemplateLoader(user=\"cms\", table=\"mail_templates\")". This string will be shown in TemplateNotFoundException exception messages, next to the template name. 
For those who has to dig deeper, note that the TemplateLoader is actually stored inside the TemplateCache of the Configuration, and is normally only accessed directly by the TemplateCache, and templates are get via the TemplateCache API-s. /**
 * FreeMarker loads template "files" through objects that implement this interface,
 * thus the templates need not be real files, and can come from any kind of data source
 * (like classpath, servlet context, database, etc). While FreeMarker provides a few
 * {@link TemplateLoader} implementations out-of-the-box, it's normal for embedding
 * frameworks to use their own implementations.
 * 
 * <p>To set the {@link TemplateLoader} used by FreeMaker, use
 * {@link Configuration#setTemplateLoader(TemplateLoader)}.
 * 
 * <p>Implementations of this interface should be thread-safe.
 * 
 * <p>Implementations should override {@link Object#toString()} to show information about from where the
 * {@link TemplateLoader} loads the templates. For example, for a template loader that loads template from database
 * table {@code toString} could return something like
 * {@code "MyDatabaseTemplateLoader(user=\"cms\", table=\"mail_templates\")"}. This string will be shown in
 * {@link TemplateNotFoundException} exception messages, next to the template name.
 * 
 * <p>For those who has to dig deeper, note that the {@link TemplateLoader} is actually stored inside
 * the {@link TemplateCache} of the {@link Configuration}, and is normally only accessed directly
 * by the {@link TemplateCache}, and templates are get via the {@link TemplateCache} API-s.
 */
public interface TemplateLoader {
	
    
Finds the template in the backing storage and returns an object that identifies the storage location where the
template can be loaded from. See the return value for more information.
Params:
name –  The name (template root directory relative path) of the template, already localized and normalized by the cache. It is completely up to the loader implementation to interpret the name, however it should expect to receive hierarchical paths where path components are separated by a slash (not backslash). Backslashes (or any other OS specific separator character) are not considered as separators by FreeMarker, and thus they will not be replaced with slash before passing to this method, so it's up to the template loader to handle them (say, by throwing an exception that tells the user that the path (s)he has entered is invalid, as (s)he must use slash -- typical mistake of Windows users). The passed names are always considered relative to some loader-defined root location (often referred as the "template root directory"), and will never start with a slash, nor will they contain a path component consisting of either a single or a double dot -- these are all resolved by the template cache before passing the name to the loader. As a side effect, paths that trivially reach outside template root directory, such as ../my.ftl, will be
           rejected by the template cache, so they never reach the template loader. Note again, that if the path
           uses backslash as path separator instead of slash as (the template loader should not accept that), the
           normalization will not properly happen, as FreeMarker (the cache) recognizes only the slashes as
           separators.
Throws:
IOException –  When an error occurs that makes it impossible to find out if the template exists, or to access the existing template. Don't throw exception if the template doesn't exist, instead return with null then!
Returns:
An object representing the template source, which can be supplied in subsequent calls to getLastModified(Object) and getReader(Object, String), when those are called on the same TemplateLoader. null must be returned if the source for the template doesn't exist; don't throw exception then! The exact type of this object is up to the TemplateLoader implementation. As this object is possibly used as hash key in caches, and is surly compared with another template source for equality, it must have a proper Object.equals(Object) and Object.hashCode()) implementation. Especially, template sources that refer to the same physical source must be equivalent, otherwise template caching can become inefficient. This is only expected from Object.equals(Object) when the compared template sources came from the same TemplateLoader instance. Also, it must not influence the equality if the source is open or closed (closeTemplateSource(Object))./**
     * Finds the template in the backing storage and returns an object that identifies the storage location where the
     * template can be loaded from. See the return value for more information.
     *
     * @param name
     *            The name (template root directory relative path) of the template, already localized and normalized by
     *            the {@link freemarker.cache.TemplateCache cache}. It is completely up to the loader implementation to
     *            interpret the name, however it should expect to receive hierarchical paths where path components are
     *            separated by a slash (not backslash). Backslashes (or any other OS specific separator character) are
     *            not considered as separators by FreeMarker, and thus they will not be replaced with slash before
     *            passing to this method, so it's up to the template loader to handle them (say, by throwing an
     *            exception that tells the user that the path (s)he has entered is invalid, as (s)he must use slash --
     *            typical mistake of Windows users). The passed names are always considered relative to some
     *            loader-defined root location (often referred as the "template root directory"), and will never start
     *            with a slash, nor will they contain a path component consisting of either a single or a double dot --
     *            these are all resolved by the template cache before passing the name to the loader. As a side effect,
     *            paths that trivially reach outside template root directory, such as <tt>../my.ftl</tt>, will be
     *            rejected by the template cache, so they never reach the template loader. Note again, that if the path
     *            uses backslash as path separator instead of slash as (the template loader should not accept that), the
     *            normalization will not properly happen, as FreeMarker (the cache) recognizes only the slashes as
     *            separators.
     *
     * @return An object representing the template source, which can be supplied in subsequent calls to
     *         {@link #getLastModified(Object)} and {@link #getReader(Object, String)}, when those are called on the
     *         same {@link TemplateLoader}. {@code null} must be returned if the source for the template doesn't exist;
     *         don't throw exception then! The exact type of this object is up to the {@link TemplateLoader}
     *         implementation. As this object is possibly used as hash key in caches, and is surly compared with another
     *         template source for equality, <b>it must have a proper {@link Object#equals(Object)} and
     *         {@link Object#hashCode()}) implementation</b>. Especially, template sources that refer to the same
     *         physical source must be equivalent, otherwise template caching can become inefficient. This is only
     *         expected from {@link Object#equals(Object)} when the compared template sources came from the same
     *         {@link TemplateLoader} instance. Also, it must not influence the equality if the source is open or
     *         closed ({@link #closeTemplateSource(Object)}). 
     * 
     * @throws IOException
     *             When an error occurs that makes it impossible to find out if the template exists, or to access the
     *             existing template. Don't throw exception if the template doesn't exist, instead return with
     *             {@code null} then!
     */
    public Object findTemplateSource(String name)
    throws IOException;
        
    
Returns the time of last modification of the specified template source. This method is called after
findTemplateSource().
Params:
templateSource –  an object representing a template source, obtained through a prior call to findTemplateSource(String). This must be an object on which closeTemplateSource(Object) wasn't applied yet.
Returns:
the time of last modification of the specified template source, or -1 if the time is not known./**
     * Returns the time of last modification of the specified template source. This method is called after
     * <code>findTemplateSource()</code>.
     * 
     * @param templateSource
     *            an object representing a template source, obtained through a prior call to
     *            {@link #findTemplateSource(String)}. This must be an object on which
     *            {@link TemplateLoader#closeTemplateSource(Object)} wasn't applied yet.
     * @return the time of last modification of the specified template source, or -1 if the time is not known.
     */
    public long getLastModified(Object templateSource);
    
    
Returns the character stream of a template represented by the specified template source. This method is possibly called for multiple times for the same template source object, and it must always return a Reader that reads the template from its beginning. Before this method is called for the second time (or later), its caller must close the previously returned Reader, and it must not use it anymore. That is, this method is not required to support multiple concurrent readers for the same source templateSource object. 
 Typically, this method is called if the template is missing from the cache, or if after calling findTemplateSource(String) and getLastModified(Object) it was determined that the cached copy of the template is stale. Then, if it turns out that the encoding parameter used doesn't match the actual template content (based on the #ftl encoding=... header), this method will be called for a second time with the correct encoding parameter value. 
 Unlike findTemplateSource(String), this method must not tolerate if the template is not found, and must throw IOException in that case. 
Params:
templateSource –  an object representing a template source, obtained through a prior call to findTemplateSource(String). This must be an object on which closeTemplateSource(Object) wasn't applied yet.
encoding – 
           the character encoding used to translate source bytes to characters. Some loaders may not have access
           to the byte representation of the template stream, and instead directly obtain a character stream.
           These loaders should ignore the encoding parameter.
Throws:
IOException – 
            if an I/O error occurs while accessing the stream.
Returns:
A Reader representing the template character stream; not null. It's the responsibility of the caller (which is TemplateCache usually) to close() it. The Reader is not required to work after the templateSource was closed (closeTemplateSource(Object))./**
     * Returns the character stream of a template represented by the specified template source. This method is possibly
     * called for multiple times for the same template source object, and it must always return a {@link Reader} that
     * reads the template from its beginning. Before this method is called for the second time (or later), its caller
     * must close the previously returned {@link Reader}, and it must not use it anymore. That is, this method is not
     * required to support multiple concurrent readers for the same source {@code templateSource} object.
     * 
     * <p>
     * Typically, this method is called if the template is missing from the cache, or if after calling
     * {@link #findTemplateSource(String)} and {@link #getLastModified(Object)} it was determined that the cached copy
     * of the template is stale. Then, if it turns out that the {@code encoding} parameter used doesn't match the actual
     * template content (based on the {@code #ftl encoding=...} header), this method will be called for a second time
     * with the correct {@code encoding} parameter value.
     * 
     * <p>
     * Unlike {@link #findTemplateSource(String)}, this method must not tolerate if the template is not found, and
     * must throw {@link IOException} in that case. 
     * 
     * @param templateSource
     *            an object representing a template source, obtained through a prior call to
     *            {@link #findTemplateSource(String)}. This must be an object on which
     *            {@link TemplateLoader#closeTemplateSource(Object)} wasn't applied yet.
     * @param encoding
     *            the character encoding used to translate source bytes to characters. Some loaders may not have access
     *            to the byte representation of the template stream, and instead directly obtain a character stream.
     *            These loaders should ignore the encoding parameter.
     * 
     * @return A {@link Reader} representing the template character stream; not {@code null}. It's the responsibility of
     *         the caller (which is {@link TemplateCache} usually) to {@code close()} it. The {@link Reader} is not
     *         required to work after the {@code templateSource} was closed ({@link #closeTemplateSource(Object)}).
     * 
     * @throws IOException
     *             if an I/O error occurs while accessing the stream.
     */
    public Reader getReader(Object templateSource, String encoding) throws IOException;
    
    
Closes the template source, releasing any resources held that are only required for reading the template and/or its metadata. This is the last method that is called by the TemplateCache for a template source, except that Object.equals(Object) is might called later too. TemplateCache ensures that this method will be called on every object that is returned from findTemplateSource(String). 
Params:
templateSource – 
           the template source that should be closed./**
     * Closes the template source, releasing any resources held that are only required for reading the template and/or
     * its metadata. This is the last method that is called by the {@link TemplateCache} for a template source, except
     * that {@link Object#equals(Object)} is might called later too. {@link TemplateCache} ensures that this method will
     * be called on every object that is returned from {@link #findTemplateSource(String)}.
     * 
     * @param templateSource
     *            the template source that should be closed.
     */
    public void closeTemplateSource(Object templateSource) throws IOException;
    
}