Question

我在addFilter（）

后过滤实体时遇到问题

已从Google App Engine弃用。

目标：列出那些具有UID =“rahul”的实体

我尝试了什么：

com.google.appengine.api.datastore.DatastoreService ds=DatastoreServiceFactory.getDatastoreService();

    Query q = new Query("Upload");
    q.setFilter(FilterOperator.EQUAL.of("UID","rahul"));
    PreparedQuery pq = ds.prepare(q);

    for (Entity result : pq.asIterable())   

       {
            String title = (String) result.getProperty("url");
            resp.getWriter().println(title);
        }

这是我的“上传”实体的DataStore实例：

Upload Entity Instance in Datastore

但是，我在启动应用程序时会看到一个空白页面。

Answer 1

不推荐query.setFilter(new Query.FilterPredicate("UID", FilterOperator.EQUAL, "rahul"));，您也可以使用以下代码：

创建单个过滤器：

new CompositeFilter(CompositeFilterOperator.AND, Arrays.asList(filter1, filter2));

创建多个过滤器：

构建完所有过滤器后，使用CompositeFilter将它们组合在一起：

query.setFilter(composite_filter);

然后通过编写以下内容将其设置为查询的过滤器：

#include <cstdio>
#include <cstdlib>
#include <algorithm>
#include <iterator>
#include <string>
#include <vector>

#include <boost/filesystem/operations.hpp>
#include <boost/filesystem/path.hpp>
#include <boost/predef.h>
#include <boost/version.hpp>
#include <boost/tokenizer.hpp>

#if (BOOST_VERSION > BOOST_VERSION_NUMBER(1,64,0))
#  include <boost/process.hpp>
#endif

#if (BOOST_OS_CYGWIN || BOOST_OS_WINDOWS)
#  include <Windows.h>
#endif

#include <boost/executable_path.hpp>
#include <boost/detail/executable_path_internals.hpp>

namespace boost {

#if (BOOST_OS_CYGWIN || BOOST_OS_WINDOWS)

std::string executable_path(const char* argv0)
{
  typedef std::vector<char> char_vector;
  typedef std::vector<char>::size_type size_type;
  char_vector buf(1024, 0);
  size_type size = buf.size();
  bool havePath = false;
  bool shouldContinue = true;
  do
  {
    DWORD result = GetModuleFileNameA(nullptr, &buf[0], size);
    DWORD lastError = GetLastError();
    if (result == 0)
    {
      shouldContinue = false;
    }
    else if (result < size)
    {
      havePath = true;
      shouldContinue = false;
    }
    else if (
      result == size
      && (lastError == ERROR_INSUFFICIENT_BUFFER || lastError == ERROR_SUCCESS)
      )
    {
      size *= 2;
      buf.resize(size);
    }
    else
    {
      shouldContinue = false;
    }
  } while (shouldContinue);
  if (!havePath)
  {
    return detail::executable_path_fallback(argv0);
  }
  // On Microsoft Windows, there is no need to call boost::filesystem::canonical or
  // boost::filesystem::path::make_preferred. The path returned by GetModuleFileNameA
  // is the one we want.
  std::string ret = &buf[0];
  return ret;
}

#elif (BOOST_OS_MACOS)

#  include <mach-o/dyld.h>

std::string executable_path(const char* argv0)
{
  typedef std::vector<char> char_vector;
  char_vector buf(1024, 0);
  uint32_t size = static_cast<uint32_t>(buf.size());
  bool havePath = false;
  bool shouldContinue = true;
  do
  {
    int result = _NSGetExecutablePath(&buf[0], &size);
    if (result == -1)
    {
      buf.resize(size + 1);
      std::fill(std::begin(buf), std::end(buf), 0);
    }
    else
    {
      shouldContinue = false;
      if (buf.at(0) != 0)
      {
        havePath = true;
      }
    }
  } while (shouldContinue);
  if (!havePath)
  {
    return detail::executable_path_fallback(argv0);
  }
  std::string path(&buf[0], size);
  boost::system::error_code ec;
  boost::filesystem::path p(
    boost::filesystem::canonical(path, boost::filesystem::current_path(), ec));
  if (ec.value() == boost::system::errc::success)
  {
    return p.make_preferred().string();
  }
  return detail::executable_path_fallback(argv0);
}

#elif (BOOST_OS_SOLARIS)

#  include <stdlib.h>

std::string executable_path(const char* argv0)
{
  std::string ret = getexecname();
  if (ret.empty())
  {
    return detail::executable_path_fallback(argv0);
  }
  boost::filesystem::path p(ret);
  if (!p.has_root_directory())
  {
    boost::system::error_code ec;
    p = boost::filesystem::canonical(
      p, boost::filesystem::current_path(), ec);
    if (ec.value() != boost::system::errc::success)
    {
      return detail::executable_path_fallback(argv0);
    }
    ret = p.make_preferred().string();
  }
  return ret;
}

#elif (BOOST_OS_BSD)

#  include <sys/sysctl.h>

std::string executable_path(const char* argv0)
{
  typedef std::vector<char> char_vector;
  int mib[4]{0};
  size_t size;
  mib[0] = CTL_KERN;
  mib[1] = KERN_PROC;
  mib[2] = KERN_PROC_PATHNAME;
  mib[3] = -1;
  int result = sysctl(mib, 4, nullptr, &size, nullptr, 0);
  if (-1 == result)
  {
    return detail::executable_path_fallback(argv0);
  }
  char_vector buf(size + 1, 0);
  result = sysctl(mib, 4, &buf[0], &size, nullptr, 0);
  if (-1 == result)
  {
    return detail::executable_path_fallback(argv0);
  }
  std::string path(&buf[0], size);
  boost::system::error_code ec;
  boost::filesystem::path p(
    boost::filesystem::canonical(
      path, boost::filesystem::current_path(), ec));
  if (ec.value() == boost::system::errc::success)
  {
    return p.make_preferred().string();
  }
  return detail::executable_path_fallback(argv0);
}

#elif (BOOST_OS_LINUX)

#  include <unistd.h>

std::string executable_path(const char *argv0)
{
  typedef std::vector<char> char_vector;
  typedef std::vector<char>::size_type size_type;
  char_vector buf(1024, 0);
  size_type size = buf.size();
  bool havePath = false;
  bool shouldContinue = true;
  do
  {
    ssize_t result = readlink("/proc/self/exe", &buf[0], size);
    if (result < 0)
    {
      shouldContinue = false;
    }
    else if (static_cast<size_type>(result) < size)
    {
      havePath = true;
      shouldContinue = false;
      size = result;
    }
    else
    {
      size *= 2;
      buf.resize(size);
      std::fill(std::begin(buf), std::end(buf), 0);
    }
  } while (shouldContinue);
  if (!havePath)
  {
    return detail::executable_path_fallback(argv0);
  }
  std::string path(&buf[0], size);
  boost::system::error_code ec;
  boost::filesystem::path p(
    boost::filesystem::canonical(
      path, boost::filesystem::current_path(), ec));
  if (ec.value() == boost::system::errc::success)
  {
    return p.make_preferred().string();
  }
  return detail::executable_path_fallback(argv0);
}

#else

std::string executable_path(const char *argv0)
{
  return detail::executable_path_fallback(argv0);
}

#endif

}

Answer 2

它应该提供所需的输出。如果它是空白页面，那么请检查Google Dashboard中的日志或共享日志。

在Google应用引擎中添加过滤器查询

2 个答案: