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Closure Compiler is a JavaScript optimizing compiler. It parses your JavaScript, analyzes it, removes dead code and rewrites and minimizes what's left. It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls. It is used in many of Google's JavaScript apps, including Gmail, Google Web Search, Google Maps, and Google Docs.

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/*
 * Copyright 2018 The Closure Compiler Authors.
 *
 * 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.google.javascript.jscomp;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.GwtIncompatible;
import com.google.common.cache.CacheBuilder;
import com.google.common.cache.CacheLoader;
import com.google.common.cache.LoadingCache;
import com.google.common.cache.RemovalNotification;
import java.io.IOException;
import java.io.InputStream;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.attribute.FileTime;
import java.util.zip.ZipEntry;
import java.util.zip.ZipFile;

/**
 * A class that abstract entries from zip files via managed caching.
 *
 * Normally zip files created via URL connection cache indefinite by JDK. That's not good since a
 * zip file contents might change over time (e.g. compiler running as a worker). This class provides
 * a timestamp controlled caching which ensures we always read up-to-date zip while avoiding wasting
 * time by re-reading the zip for each entry.
 */
@GwtIncompatible("java.util.zip.ZipFile")
final class JSCompZipFileCache {

  /**
   * Return a stream for an entry.
   *
   * We expose a stream rather than something like a ZipEntry to avoid possible cache-freshness
   * issues. Callers must read the stream "quickly" and may only do so once.
   */
  static InputStream getEntryStream(String zipName, String entryName) throws IOException {
    return zipFileCache.getUnchecked(zipName).getEntryStream(entryName);
  }

  private static final int ZIP_CACHE_SIZE =
      Integer.parseInt(System.getProperty("jscomp.zipfile.cachesize", "1000"));

  private static final LoadingCache zipFileCache =
      CacheBuilder.newBuilder()
          .maximumSize(ZIP_CACHE_SIZE)
          .removalListener(
              (RemovalNotification notification) -> {
                try {
                  notification.getValue().maybeClose();
                } catch (IOException e) {
                  throw new RuntimeException(e);
                }
              })
          .build(
              new CacheLoader() {
                @Override
                public CachedZipFile load(String key) {
                  return new CachedZipFile(key);
                }
              });

  private static final class CachedZipFile {
    private final Path path;
    private ZipFile zipFile;
    private volatile FileTime lastModified;

    private CachedZipFile(String zipName) {
      this.path = Paths.get(zipName);
    }

    InputStream getEntryStream(String entryName) throws IOException {
      refreshIfNeeded();
      ZipEntry entry = checkNotNull(this.zipFile.getEntry(entryName), "%s!/%s", path, entryName);
      return this.zipFile.getInputStream(entry);
    }

    private void refreshIfNeeded() throws IOException {
      FileTime newLastModified = Files.getLastModifiedTime(path);
      if (newLastModified.equals(lastModified)) {
        return;
      }

      synchronized (this) {
        // Since we do double checked locking (newLastModified is checked out of synchronized), we
        // should test the stamp again.
        if (newLastModified.equals(lastModified)) {
          return;
        }

        maybeClose();
        zipFile = new ZipFile(path.toFile());
        lastModified = newLastModified;
      }
    }

    private void maybeClose() throws IOException {
      if (zipFile != null) {
        zipFile.close();
      }
    }
  }

  private JSCompZipFileCache() {
    throw new AssertionError();
  }
}