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/*
* Copyright (C) 2016 The Android Open Source Project
*
* Licensed 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.
*/
package com.android.apkzlib.utils;
import com.google.common.base.Objects;
import com.google.common.hash.HashCode;
import com.google.common.hash.Hashing;
import com.google.common.io.Files;
import java.io.File;
import java.io.IOException;
import javax.annotation.Nonnull;
import javax.annotation.Nullable;
/**
* A cache for file contents. The cache allows closing a file and saving in memory its contents (or
* some related information). It can then be used to check if the contents are still valid at some
* later time. Typical usage flow is:
*
* <p>
*
* <pre>{@code
* Object fileRepresentation = // ...
* File toWrite = // ...
* // Write file contents and update in memory representation
* CachedFileContents<Object> contents = new CachedFileContents<Object>(toWrite);
* contents.closed(fileRepresentation);
*
* // Later, when data is needed:
* if (contents.isValid()) {
* fileRepresentation = contents.getCache();
* } else {
* // Re-read the file and recreate the file representation
* }
* }</pre>
*
* @param <T> the type of cached contents
*/
public class CachedFileContents<T> {
/**
* The file.
*/
@Nonnull
private File file;
/**
* Time when last closed (time when {@link #closed(Object)} was invoked).
*/
private long lastClosed;
/**
* Size of the file when last closed.
*/
private long size;
/**
* Hash of the file when closed. {@code null} if hashing failed for some reason.
*/
@Nullable
private HashCode hash;
/**
* Cached data associated with the file.
*/
@Nullable
private T cache;
/**
* Creates a new contents. When the file is written, {@link #closed(Object)} should be invoked
* to set the cache.
*
* @param file the file
*/
public CachedFileContents(@Nonnull File file) {
this.file = file;
}
/**
* Should be called when the file's contents are set and the file closed. This will save the
* cache and register the file's timestamp to later detect if it has been modified.
* <p>
* This method can be called as many times as the file has been written.
*
* @param cache an optional cache to save
*/
public void closed(@Nullable T cache) {
this.cache = cache;
lastClosed = file.lastModified();
size = file.length();
hash = hashFile();
}
/**
* Are the cached contents still valid? If this method determines that the file has been
* modified since the last time {@link #closed(Object)} was invoked.
*
* @return are the cached contents still valid? If this method returns {@code false}, the
* cache is cleared
*/
public boolean isValid() {
boolean valid = true;
if (!file.exists()) {
valid = false;
}
if (valid && file.lastModified() != lastClosed) {
valid = false;
}
if (valid && file.length() != size) {
valid = false;
}
if (valid && !Objects.equal(hash, hashFile())) {
valid = false;
}
if (!valid) {
cache = null;
}
return valid;
}
/**
* Obtains the cached data set with {@link #closed(Object)} if the file has not been modified
* since {@link #closed(Object)} was invoked.
*
* @return the last cached data or {@code null} if the file has been modified since
* {@link #closed(Object)} has been invoked
*/
@Nullable
public T getCache() {
return cache;
}
/**
* Computes the hashcode of the cached file.
*
* @return the hash code
*/
@Nullable
private HashCode hashFile() {
try {
return Files.hash(file, Hashing.crc32());
} catch (IOException e) {
return null;
}
}
/**
* Obtains the file used for caching.
*
* @return the file; this file always exists and contains the old (cached) contents of the
* file
*/
@Nonnull
public File getFile() {
return file;
}
}
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