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diff --git a/tools/java/runfiles/Runfiles.java b/tools/java/runfiles/Runfiles.java
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+// Copyright 2018 The Bazel Authors. All rights reserved.
+//
+// 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.devtools.build.runfiles;
+
+import java.io.BufferedReader;
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.nio.charset.StandardCharsets;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Runfiles lookup library for Bazel-built Java binaries and tests.
+ *
+ * <p>USAGE:
+ *
+ * <p>1. Depend on this runfiles library from your build rule:
+ *
+ * <pre>
+ *   java_binary(
+ *       name = "my_binary",
+ *       ...
+ *       deps = ["@bazel_tools//tools/java/runfiles"],
+ *   )
+ * </pre>
+ *
+ * <p>2. Import the runfiles library.
+ *
+ * <pre>
+ *   import com.google.devtools.build.runfiles.Runfiles;
+ * </pre>
+ *
+ * <p>3. Create a Runfiles object and use rlocation to look up runfile paths:
+ *
+ * <pre>
+ *   public void myFunction() {
+ *     Runfiles runfiles = Runfiles.create();
+ *     String path = runfiles.rlocation("my_workspace/path/to/my/data.txt");
+ *     ...
+ * </pre>
+ *
+ * <p>If you want to start subprocesses that also need runfiles, you need to set the right
+ * environment variables for them:
+ *
+ * <pre>
+ *   String path = r.rlocation("path/to/binary");
+ *   ProcessBuilder pb = new ProcessBuilder(path);
+ *   pb.environment().putAll(r.getEnvVars());
+ *   ...
+ *   Process p = pb.start();
+ * </pre>
+ */
+public abstract class Runfiles {
+
+  // Package-private constructor, so only package-private classes may extend it.
+  private Runfiles() {}
+
+  /**
+   * Returns a new {@link Runfiles} instance.
+   *
+   * <p>This method passes the JVM's environment variable map to {@link #create(Map)}.
+   */
+  public static Runfiles create() throws IOException {
+    return create(System.getenv());
+  }
+
+  /**
+   * Returns a new {@link Runfiles} instance.
+   *
+   * <p>The returned object is either:
+   *
+   * <ul>
+   *   <li>manifest-based, meaning it looks up runfile paths from a manifest file, or
+   *   <li>directory-based, meaning it looks up runfile paths under a given directory path
+   * </ul>
+   *
+   * <p>If {@code env} contains "RUNFILES_MANIFEST_ONLY" with value "1", this method returns a
+   * manifest-based implementation. The manifest's path is defined by the "RUNFILES_MANIFEST_FILE"
+   * key's value in {@code env}.
+   *
+   * <p>Otherwise this method returns a directory-based implementation. The directory's path is
+   * defined by the value in {@code env} under the "RUNFILES_DIR" key, or if absent, then under the
+   * "JAVA_RUNFILES" key.
+   *
+   * <p>Note about performance: the manifest-based implementation eagerly reads and caches the whole
+   * manifest file upon instantiation.
+   *
+   * @throws IOException if RUNFILES_MANIFEST_ONLY=1 is in {@code env} but there's no
+   *     "RUNFILES_MANIFEST_FILE", "RUNFILES_DIR", or "JAVA_RUNFILES" key in {@code env} or their
+   *     values are empty, or some IO error occurs
+   */
+  public static Runfiles create(Map<String, String> env) throws IOException {
+    if (isManifestOnly(env)) {
+      // On Windows, Bazel sets RUNFILES_MANIFEST_ONLY=1.
+      // On every platform, Bazel also sets RUNFILES_MANIFEST_FILE, but on Linux and macOS it's
+      // faster to use RUNFILES_DIR.
+      return new ManifestBased(getManifestPath(env));
+    } else {
+      return new DirectoryBased(getRunfilesDir(env));
+    }
+  }
+
+  /**
+   * Returns the runtime path of a runfile (a Bazel-built binary's/test's data-dependency).
+   *
+   * <p>The returned path may not be valid. The caller should check the path's validity and that the
+   * path exists.
+   *
+   * <p>The function may return null. In that case the caller can be sure that the rule does not
+   * know about this data-dependency.
+   *
+   * @param path runfiles-root-relative path of the runfile
+   * @throws IllegalArgumentException if {@code path} fails validation, for example if it's null or
+   *     empty, or not normalized (contains "./", "../", or "//")
+   */
+  public final String rlocation(String path) {
+    Util.checkArgument(path != null);
+    Util.checkArgument(!path.isEmpty());
+    Util.checkArgument(
+        !path.startsWith("../")
+            && !path.contains("/..")
+            && !path.startsWith("./")
+            && !path.contains("/./")
+            && !path.endsWith("/.")
+            && !path.contains("//"),
+        "path is not normalized: \"%s\"",
+        path);
+    Util.checkArgument(
+        !path.startsWith("\\"), "path is absolute without a drive letter: \"%s\"", path);
+    if (new File(path).isAbsolute()) {
+      return path;
+    }
+    return rlocationChecked(path);
+  }
+
+  /**
+   * Returns environment variables for subprocesses.
+   *
+   * <p>The caller should add the returned key-value pairs to the environment of subprocesses in
+   * case those subprocesses are also Bazel-built binaries that need to use runfiles.
+   */
+  public abstract Map<String, String> getEnvVars();
+
+  /** Returns true if the platform supports runfiles only via manifests. */
+  private static boolean isManifestOnly(Map<String, String> env) {
+    return "1".equals(env.get("RUNFILES_MANIFEST_ONLY"));
+  }
+
+  private static String getManifestPath(Map<String, String> env) throws IOException {
+    String value = env.get("RUNFILES_MANIFEST_FILE");
+    if (Util.isNullOrEmpty(value)) {
+      throw new IOException(
+          "Cannot load runfiles manifest: $RUNFILES_MANIFEST_ONLY is 1 but"
+              + " $RUNFILES_MANIFEST_FILE is empty or undefined");
+    }
+    return value;
+  }
+
+  private static String getRunfilesDir(Map<String, String> env) throws IOException {
+    String value = env.get("RUNFILES_DIR");
+    if (Util.isNullOrEmpty(value)) {
+      value = env.get("JAVA_RUNFILES");
+    }
+    if (Util.isNullOrEmpty(value)) {
+      throw new IOException(
+          "Cannot find runfiles: $RUNFILES_DIR and $JAVA_RUNFILES are both unset or empty");
+    }
+    return value;
+  }
+
+  abstract String rlocationChecked(String path);
+
+  /** {@link Runfiles} implementation that parses a runfiles-manifest file to look up runfiles. */
+  private static final class ManifestBased extends Runfiles {
+    private final Map<String, String> runfiles;
+    private final String manifestPath;
+
+    ManifestBased(String manifestPath) throws IOException {
+      Util.checkArgument(manifestPath != null);
+      Util.checkArgument(!manifestPath.isEmpty());
+      this.manifestPath = manifestPath;
+      this.runfiles = loadRunfiles(manifestPath);
+    }
+
+    private static Map<String, String> loadRunfiles(String path) throws IOException {
+      HashMap<String, String> result = new HashMap<>();
+      try (BufferedReader r =
+          new BufferedReader(
+              new InputStreamReader(new FileInputStream(path), StandardCharsets.UTF_8))) {
+        String line = null;
+        while ((line = r.readLine()) != null) {
+          int index = line.indexOf(' ');
+          String runfile = (index == -1) ? line : line.substring(0, index);
+          String realPath = (index == -1) ? line : line.substring(index + 1);
+          result.put(runfile, realPath);
+        }
+      }
+      return Collections.unmodifiableMap(result);
+    }
+
+    private static String findRunfilesDir(String manifest) {
+      if (manifest.endsWith("/MANIFEST")
+          || manifest.endsWith("\\MANIFEST")
+          || manifest.endsWith(".runfiles_manifest")) {
+        String path = manifest.substring(0, manifest.length() - 9);
+        if (new File(path).isDirectory()) {
+          return path;
+        }
+      }
+      return "";
+    }
+
+    @Override
+    public String rlocationChecked(String path) {
+      return runfiles.get(path);
+    }
+
+    @Override
+    public Map<String, String> getEnvVars() {
+      HashMap<String, String> result = new HashMap<>(4);
+      result.put("RUNFILES_MANIFEST_ONLY", "1");
+      result.put("RUNFILES_MANIFEST_FILE", manifestPath);
+      String runfilesDir = findRunfilesDir(manifestPath);
+      result.put("RUNFILES_DIR", runfilesDir);
+      // TODO(laszlocsomor): remove JAVA_RUNFILES once the Java launcher can pick up RUNFILES_DIR.
+      result.put("JAVA_RUNFILES", runfilesDir);
+      return result;
+    }
+  }
+
+  /** {@link Runfiles} implementation that appends runfiles paths to the runfiles root. */
+  private static final class DirectoryBased extends Runfiles {
+    private final String runfilesRoot;
+
+    DirectoryBased(String runfilesDir) throws IOException {
+      Util.checkArgument(!Util.isNullOrEmpty(runfilesDir));
+      Util.checkArgument(new File(runfilesDir).isDirectory());
+      this.runfilesRoot = runfilesDir;
+    }
+
+    @Override
+    String rlocationChecked(String path) {
+      return runfilesRoot + "/" + path;
+    }
+
+    @Override
+    public Map<String, String> getEnvVars() {
+      HashMap<String, String> result = new HashMap<>(2);
+      result.put("RUNFILES_DIR", runfilesRoot);
+      // TODO(laszlocsomor): remove JAVA_RUNFILES once the Java launcher can pick up RUNFILES_DIR.
+      result.put("JAVA_RUNFILES", runfilesRoot);
+      return result;
+    }
+  }
+
+  static Runfiles createManifestBasedForTesting(String manifestPath) throws IOException {
+    return new ManifestBased(manifestPath);
+  }
+
+  static Runfiles createDirectoryBasedForTesting(String runfilesDir) throws IOException {
+    return new DirectoryBased(runfilesDir);
+  }
+}