非 public 构造函数导致 Tomcat7 出现问题？

Question

我在 tomcat 上有 Java 应用 Spring 运行。

这个 class 给我带来了一个非常奇怪的问题：

@WebListener
public class ThreadPool extends ThreadPoolExecutor implements ServletContextListener {

  private ThreadPool() {
    super(MIN_ACTIVE_THREADS, MAX_ACTIVE_THREADS, DEACTIVATE_THREADS_AFTER_TIMEPERIOD, TimeUnit.SECONDS, taskQueue);
  }

  private static final ThreadPoolExecutor pool = new ThreadPool();

  public synchronized static void submit(Task task) {
      executingTasks.add(task);
      pool.execute(task);
  }

  @Override
  public synchronized void contextDestroyed(ServletContextEvent arg0) {
    cancelWaitingTasks();
    sendStopSignalsToExecutingTasks();
    pool.shutdown();
  }

  ...

}

如果构造函数是私有的或默认的，我会在运行时遇到此异常（在对应用程序的第一个 HTTP 请求时）：

Error configuring application listener of class com.testApp.util.ThreadPool
java.lang.IllegalAccessException: Class org.apache.catalina.core.DefaultInstanceManager can not access a member of class com.testApp.util.ThreadPool with modifiers "private"
    at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:102)
    at java.lang.Class.newInstance(Class.java:436)
    at org.apache.catalina.core.DefaultInstanceManager.newInstance(DefaultInstanceManager.java:140)
    at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4888)
    at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5467)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
    at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1559)
    at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1549)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)
Skipped installing application listeners due to previous error(s)
Error listenerStart
Context [] startup failed due to previous errors

但是，如果我设置构造函数 public，那么我不会出现任何异常，并且一切正常。谁能告诉我为什么这个默认或私有构造函数会导致运行时异常？

Answer 1

通过报错，说的很清楚是因为无法访问class的成员。

can not access a member of class com.testApp.util.ThreadPool with modifiers

Answer 2

Tomcat 使用 Class.newInstance() 创建您的 ThreadPool 的实例。此方法遵守 Java 的访问规则。

由于您的构造函数是私有的，因此它失败并显示 IllegalAccessException。这是运行时等效于不允许调用编译器错误的函数，如果您尝试在 ThreadPool、

之外编写 new ThreadPool()，您会看到该错误

Answer 3

Tomcat 的 org.apache.catalina.core.DefaultInstanceManager 正在尝试创建您的 ThreadPool 的对象，您已将其配置为上下文侦听器。现在，由于这是在 org.apache.catalina.core 之外，您必须使用 public 构造函数，否则 org.apache.catalina.core.DefaultInstanceManager 将无法创建其对象。

来自org.apache.catalina.core.DefaultInstanceManager

private Object newInstance(Object instance, Class<?> clazz) throws IllegalAccessException, InvocationTargetException, NamingException {
    if (!ignoreAnnotations) {
        Map<String, String> injections = injectionMap.get(clazz.getName());
        processAnnotations(instance, injections);
        postConstruct(instance, clazz);
    }
    return instance;
}

Answer 4

我想我无意中发现了出现异常的真正原因。目前我正在使用这个 class，没有抛出异常，在 GlassFish 和 Tomcat:

上测试

public class TrackingThreadPool extends ThreadPoolExecutor {

    private static final int MAX_WAITING_TASKS = 4000;
    private static final int MAX_ACTIVE_THREADS = 20;
    private static final int MIN_ACTIVE_THREADS = 4;
    private static final int DEACTIVATE_THREADS_AFTER_SECONDS = 60;

    private TrackingThreadPool() {
        super(MIN_ACTIVE_THREADS, MAX_ACTIVE_THREADS, DEACTIVATE_THREADS_AFTER_SECONDS,
                TimeUnit.SECONDS, waitingTasks);
    }

    private static final BlockingQueue<Runnable> waitingTasks = new LinkedBlockingQueue<>(MAX_WAITING_TASKS);

    private static final Map<Long, Task> executingTasks = new HashMap<>(MAX_ACTIVE_THREADS * 2);

    private static final TrackingThreadPool instance = new TrackingThreadPool();

    public synchronized static void submitAndTrack(Task task) {
        executingTasks.put(task.getId(), task);
        instance.execute(task);
    }

    public synchronized static void shutdownAndCancelAllTasks() {
        cancelWaitingTasks();
        sendStopSignalToExecutingTasks();
        instance.shutdown();
    }

    @Override
    protected void afterExecute(Runnable r, Throwable t) {
        super.afterExecute(r, t);
        if (r instanceof Task) {
            executingTasks.remove(((Task) r).getId());
        }
    }

private static void cancelWaitingTasks() {
    List<Runnable> waitingTaskListRunnables = new ArrayList<>(waitingTasks.size() + 10); //+10 to avoid resizing
    waitingTasks.drainTo(waitingTaskListRunnables);
    for (Runnable r : waitingTaskListRunnables) {
        if (r instanceof Task) {
            ((Task) r).sendStopSignal(byShutdownMethod());
        }
    }
}

    private static void sendStopSignalToExecutingTasks() {
        for (long taskId : executingTasks.keySet()) {
            Task executingTask = executingTasks.get(taskId);
            executingTask.sendStopSignal(byShutdownMethod());
        }
    }

    private static String byShutdownMethod() {
        return TrackingThreadPool.class.getSimpleName() + "#shutdownAndCancelAllTasks()";
    }
}

如果我像这样交换 BlockingQueue<Runnable> waitingTasks 和 TrackingThreadPool instance 的位置：

private static final TrackingThreadPool instance = new TrackingThreadPool();

private static final Map<Long, Task> executingTasks = new HashMap<>(MAX_ACTIVE_THREADS * 2);

private static final BlockingQueue<Runnable> waitingTasks = new LinkedBlockingQueue<>(MAX_WAITING_TASKS);

我再次遇到异常，因为在我创建新的 TrackingThreadPool 实例时 waitingTasks 尚未实例化。

我想你可以有一个 ThreadPoolExecutor 的子 class 和一个私有构造函数/singelton 模式。

非 public 构造函数导致 Tomcat7 出现问题？

Non-public constructor causing problems with Tomcat7?

java

spring

tomcat

runtime-error