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/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file 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.
*/
package org.apache.catalina.webresources;
import java.util.BitSet;
import java.util.Enumeration;
import java.util.jar.JarEntry;
import java.util.jar.JarFile;
/**
* This class represents the contents of a jar by determining whether a given
* resource might be in the cache, based on a bloom filter. This is not a
* general-purpose bloom filter because it contains logic to strip out
* characters from the beginning of the key.
*
* The hash methods are simple but good enough for this purpose.
*/
public final class JarContents {
private final BitSet bits1;
private final BitSet bits2;
/**
* Constant used by a typical hashing method.
*/
private static final int HASH_PRIME_1 = 31;
/**
* Constant used by a typical hashing method.
*/
private static final int HASH_PRIME_2 = 17;
/**
* Size of the fixed-length bit table. Larger reduces false positives,
* smaller saves memory.
*/
private static final int TABLE_SIZE = 2048;
/**
* Parses the passed-in jar and populates the bit array.
*
* @param jar the JAR file
*/
public JarContents(JarFile jar) {
Enumeration entries = jar.entries();
bits1 = new BitSet(TABLE_SIZE);
bits2 = new BitSet(TABLE_SIZE);
while (entries.hasMoreElements()) {
JarEntry entry = entries.nextElement();
String name = entry.getName();
int startPos = 0;
// If the path starts with a slash, that's not useful information.
// Skipping it increases the significance of our key by
// removing an insignificant character.
boolean precedingSlash = name.charAt(0) == '/';
if (precedingSlash) {
startPos = 1;
}
// Find the correct table slot
int pathHash1 = hashcode(name, startPos, HASH_PRIME_1);
int pathHash2 = hashcode(name, startPos, HASH_PRIME_2);
bits1.set(pathHash1 % TABLE_SIZE);
bits2.set(pathHash2 % TABLE_SIZE);
// While directory entry names always end in "/", application code
// may look them up without the trailing "/". Add this second form.
if (entry.isDirectory()) {
pathHash1 = hashcode(name, startPos, name.length() - 1, HASH_PRIME_1);
pathHash2 = hashcode(name, startPos, name.length() - 1, HASH_PRIME_2);
bits1.set(pathHash1 % TABLE_SIZE);
bits2.set(pathHash2 % TABLE_SIZE);
}
}
}
/**
* Simple hashcode of a portion of the string. Typically we would use
* substring, but memory and runtime speed are critical.
*
* @param content
* Wrapping String.
* @param startPos
* First character in the range.
* @return hashcode of the range.
*/
private int hashcode(String content, int startPos, int hashPrime) {
return hashcode(content, startPos, content.length(), hashPrime);
}
private int hashcode(String content, int startPos, int endPos, int hashPrime) {
int h = hashPrime/2;
for (int i = startPos; i < endPos; i++) {
h = hashPrime * h + content.charAt(i);
}
if (h < 0) {
h = h * -1;
}
return h;
}
/**
* Method that identifies whether a given path MIGHT be in this jar.
* Uses the Bloom filter mechanism.
*
* @param path
* Requested path. Sometimes starts with "/WEB-INF/classes".
* @param webappRoot
* The value of the webapp location, which can be stripped from
* the path. Typically is "/WEB-INF/classes".
* @return Whether the prefix of the path is known to be in this jar.
*/
public final boolean mightContainResource(String path, String webappRoot) {
int startPos = 0;
if (path.startsWith(webappRoot)) {
startPos = webappRoot.length();
}
if (path.charAt(startPos) == '/') {
// ignore leading slash
startPos++;
}
// calculate the hash lazily and return a boolean value for this path
return (bits1.get(hashcode(path, startPos, HASH_PRIME_1) % TABLE_SIZE) &&
bits2.get(hashcode(path, startPos, HASH_PRIME_2) % TABLE_SIZE));
}
}
