FSchumacher commented on a change in pull request #352:
URL: https://github.com/apache/tomcat/pull/352#discussion_r489610355
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File path: java/org/apache/catalina/webresources/JarContents.java
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@@ -0,0 +1,122 @@
+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 <b>might</b> 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
+ */
+ public JarContents(JarFile jar) {
+ Enumeration<JarEntry> entries = jar.entries();
+ bits1 = new BitSet(TABLE_SIZE);
+ bits2 = new BitSet(TABLE_SIZE);
+
+ // Enumerations. When will they update this API?!
+ 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);
+ }
+ }
+
+ /**
+ * 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) {
+ int h = hashPrime/2;
+ int contentLength = content.length();
+ for (int i = startPos; i < contentLength; i++) {
+ h = hashPrime * h + content.charAt(i);
+ }
+
+ if (h < 0) {
+ h = h * -1;
+ }
+ return h;
+ }
+
+
+ /**
+ * Method that identifies whether a given path <b>MIGHT</b> 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++;
+ }
+
+ // Find the correct table slot
+ int pathHash1 = hashcode(path, startPos, HASH_PRIME_1);
+ int pathHash2 = hashcode(path, startPos, HASH_PRIME_2);
+ boolean probablyPresent = (bits1.get(pathHash1 % TABLE_SIZE) &&
bits2.get(pathHash2 % TABLE_SIZE));
Review comment:
Wouldn't it be even faster, when you calculate pathHash2 lazyly, i.e.
inlining the two int variables and use the fact, that the `&&` operator is lazy?
The parenthesis seem to be not needed here, too.
And last, why not return the result directly?
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