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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 2015 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.checkState;
import static com.google.common.base.Throwables.throwIfUnchecked;
import static java.util.concurrent.TimeUnit.SECONDS;

import com.google.common.annotations.GwtIncompatible;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeoutException;
import org.jspecify.nullness.Nullable;

/** Run the compiler in a separate thread with a larger stack */
class CompilerExecutor {
  // We use many recursive algorithms that use O(d) memory in the depth
  // of the tree.
  // Also, (de)serialization between phases can involve a lot of recursion.
  static final long COMPILER_STACK_SIZE = (1 << 26); // About 64MB

  /** Use a dedicated compiler thread per Compiler instance. */
  private @Nullable Thread compilerThread = null;

  /** Whether to use threads. */
  private boolean useThreads = true;

  private int timeout = 0;

  /**
   * Under JRE 1.6, the JS Compiler overflows the stack when running on some
   * large or complex JS code. When threads are available, we run all compile
   * jobs on a separate thread with a larger stack.
   *
   * That way, we don't have to increase the stack size for *every* thread
   * (which is what -Xss does).
   */
  @GwtIncompatible("java.util.concurrent.ExecutorService")
  ExecutorService getExecutorService() {
    return getDefaultExecutorService();
  }

  static ExecutorService getDefaultExecutorService() {
    return Executors.newSingleThreadExecutor(new ThreadFactory() {
      @Override
      public Thread newThread(Runnable r) {
        Thread t = new Thread(null, r, "jscompiler", COMPILER_STACK_SIZE);
        t.setDaemon(true);  // Do not prevent the JVM from exiting.
        return t;
      }
    });
  }

  void disableThreads() {
    useThreads = false;
  }

  void setTimeout(int timeout) {
    this.timeout = timeout;
  }

  @SuppressWarnings("unchecked")
   T runInCompilerThread(final Callable callable, final boolean dumpTraceReport) {
    ExecutorService executor = getExecutorService();
    T result = null;
    final Throwable[] exception = new Throwable[1];

    checkState(
        compilerThread == null || compilerThread == Thread.currentThread(),
        "Please do not share the Compiler across threads");

    // If the compiler thread is available, use it.
    if (useThreads && compilerThread == null) {
      try {
        Callable bootCompilerThread =
            () -> {
              try {
                compilerThread = Thread.currentThread();
                if (dumpTraceReport) {
                  Tracer.initCurrentThreadTrace();
                }
                return callable.call();
              } catch (Throwable e) {
                exception[0] = e;
              } finally {
                compilerThread = null;
                if (dumpTraceReport) {
                  Tracer.logCurrentThreadTrace();
                }
                Tracer.clearCurrentThreadTrace();
              }
              return null;
            };

        Future future = executor.submit(bootCompilerThread);
        if (timeout > 0) {
          result = future.get(timeout, SECONDS);
        } else {
          result = future.get();
        }
      } catch (InterruptedException | TimeoutException | ExecutionException e) {
        throw new RuntimeException(e);
      } finally {
        executor.shutdown();
      }
    } else {
      try {
        result = callable.call();
      } catch (Exception e) {
        exception[0] = e;
      }
    }

    // Pass on any exception caught by the runnable object.
    if (exception[0] != null) {
      throwIfUnchecked(exception[0]);
      throw new RuntimeException(exception[0]);
    }

    return result;
  }
}