Java 返回为null的图像Servlet图像
您好,我尝试了以下这个非常有用的图坦卡蒙在这里发现: 我按照教程尝试创建我的图像servlet,它从数据库中检索图像并将其显示在我的jsp页面上。据我所知,代码看起来正常,但控制台显示一个500内部错误,我的服务器控制台在这里指示我的Image变量为null:Java 返回为null的图像Servlet图像,java,sql,image,oracle,servlets,Java,Sql,Image,Oracle,Servlets,您好,我尝试了以下这个非常有用的图坦卡蒙在这里发现: 我按照教程尝试创建我的图像servlet,它从数据库中检索图像并将其显示在我的jsp页面上。据我所知,代码看起来正常,但控制台显示一个500内部错误,我的服务器控制台在这里指示我的Image变量为null: Image image = dataManager.getPhotos(homeId); 以下是我的完整代码: @WebServlet(name = "ImageServlet", urlPatterns = {"/Image"})
Image image = dataManager.getPhotos(homeId);
以下是我的完整代码:
@WebServlet(name = "ImageServlet", urlPatterns = {"/Image"})
public class ImageServlet extends HttpServlet {
private static final int DEFAULT_BUFFER_SIZE = 10240; // 10KB.
private static DatabaseConnector dataManager;
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
// Get ID from request.
String homeId = request.getParameter("id");
// Check if ID is supplied to the request.
if (homeId == null) {
// Do your thing if the ID is not supplied to the request.
// Throw an exception, or send 404, or show default/warning image, or just ignore it.
response.sendError(HttpServletResponse.SC_NOT_FOUND); // 404.
return;
}
try {
// Lookup Image by ImageId in database.
// Do your "SELECT * FROM Image WHERE ImageID" thing.
Image image = dataManager.getPhotos(homeId);
// Check if image is actually retrieved from database.
if (image == null) {
// Do your thing if the image does not exist in database.
// Throw an exception, or send 404, or show default/warning image, or just ignore it.
response.sendError(HttpServletResponse.SC_NOT_FOUND); // 404.
return;
}
// Init servlet response.
response.reset();
response.setBufferSize(DEFAULT_BUFFER_SIZE);
response.setContentType("image/png");
response.setHeader("Content-Length", String.valueOf(image.getLength()));
response.setHeader("Content-Disposition", "inline; filename=\"" + image.getHomeID() + "\"");
// Prepare streams.
BufferedInputStream input = null;
BufferedOutputStream output = null;
try {
// Open streams.
input = new BufferedInputStream(image.getContent(), DEFAULT_BUFFER_SIZE);
output = new BufferedOutputStream(response.getOutputStream(), DEFAULT_BUFFER_SIZE);
// Write file contents to response.
byte[] buffer = new byte[DEFAULT_BUFFER_SIZE];
int length;
while ((length = input.read(buffer)) > 0) {
output.write(buffer, 0, length);
}
} finally {
// Gently close streams.
close(output);
close(input);
}
} catch (IllegalArgumentException ex) {
Logger.getLogger(ImageServlet.class.getName()).log(Level.SEVERE, null, ex);
} catch (SQLException ex) {
Logger.getLogger(ImageServlet.class.getName()).log(Level.SEVERE, null, ex);
} catch (ClassNotFoundException ex) {
Logger.getLogger(ImageServlet.class.getName()).log(Level.SEVERE, null, ex);
}
}
private static void close(Closeable resource) {
if (resource != null) {
try {
resource.close();
} catch (IOException e) {
// Do your thing with the exception. Print it, log it or mail it.
e.printStackTrace();
}
}
}
下面的“我的数据库连接器”处理并检索以下数据:
public Image getPhotos(String homeID) throws
IllegalArgumentException, SQLException, ClassNotFoundException {
createConnection();
Image img = new Image();
ResultSet rs = null;
PreparedStatement preparedStatement = null;
String strQuery = "SELECT * "
+ "FROM home_photo "
+ "WHERE home_photo.home_id = ?";
try {
preparedStatement = conn.prepareStatement(strQuery);
preparedStatement.setString(1, homeID);
rs = preparedStatement.executeQuery();
while (rs.next()) {
img.setHomeID(rs.getInt("home_id"));
img.setPhotoID(rs.getInt("photo_id"));
img.setContent(rs.getBinaryStream("photo"));
img.setDescription(rs.getString("description"));
img.setLength(rs.getInt("length"));
img.setContentType(rs.getString("type"));
}
} catch (SQLException e) {
throw new SQLException(e);
}
closeConnection();
return img;
}
我看不出它在哪里返回img和null
感谢您的帮助。问题和代码表明,实际上是
数据管理器
为null
。代码的编写方式是,如果图像
曾经是null
,那么它只会返回一个404,而不是带有NullPointerException的500
您需要确保dataManager
不是null
,例如,通过在init()
方法中实例化它,或者如果它是一个EJB,通过将它作为@EJB
注入
与具体问题无关,DatabaseConnector
是一个静态
变量,而JDBC连接
被声明为实例变量也不是一个好迹象。此代码不是线程安全的。从这里开始学习如何编写正确的JDBC代码:在您找到image servlet的同一个博客上,您还可以找到一个basic。您好,谢谢您的回复。我使用了int()方法来安装dataManager。但是代码现在在我的控制台中Servlet中的while循环中触发了一个“Stream Closed”异常,这实际上是一个不同的问题,您应该问一个新问题:)但是,您需要用byte[]content
替换InputStream内容
,并使用ResultSet#getBytes()
来获取它。这基本上是由于关闭连接后,InputStream
不再可用造成的。