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Blob Handling in Java and JDBC

October 5, 2015
By Lorenzo Dee
Reading Time : 4
 minute
s
Blob Handling In Java and JDBC banner
Back to Home

Article

Blob Handling in Java and JDBC

October 5, 2015
By Lorenzo Dee
Reading Time : 4
 minute
s

Lorenzo simplifies BLOB handling. Read on to find out how.

Whenever I search the web for storing and retrieving BLOBs to and from a database, I usually get a complicated sample that uses byte arrays (byte[]) and JDBC code. Since I usually deal with web-based enterprise applications, I need to store BLOBs (e.g. images, spreadsheets, documents, PDFs) to a database and retrieve them later (e.g. as an <img> tag in an HTML page, or downloaded from a URL).

JDBC and BLOBs

The typical way of storing BLOBs with JDBC is shown below. Note that the created objects need to be closed properly to prevent resource leaks.

Connection connection = ...getConnection();
try {
  PreparedStatement stmt =
        connection.prepareStatement("INSERT...");
  try {
    // Sometimes, a byte[] is used here :(
    InputStream bytes = new FileInputStream("..."); // e.g. image file
    try {
      ... // other parameters set
      // Unsure on how to create a Blob and use setBlob(...)
      stmt.setBinaryStream(..., bytes);
      stmt.execute();
    } finally {
      bytes.close();
    }
  } finally {
    stmt.close();
  }
} finally {
  connection.close();
}

Note that the code above is possibly an over-simplification. Most often, the BLOB is embedded with another persistent object (e.g. a Person with a photo that is stored as a BLOB).

To retrieve the BLOB, the following code is typically seen. Note that the BLOB is only available while the JDBC connection stays open. Thus, some developers resort to using byte arrays (byte[]) to temporarily store and transfer them, before the connection gets closed.

Connection connection = ...getConnection();
try {
  PreparedStatement stmt =
        connection.prepareStatement("SELECT...");
  try {
    ... // other parameters set
    ResultSet rs = stmt.executeQuery();
    try {
      InputStream bytes = rs.getBinaryStream(...);
      // Sometimes, a byte[] is used with rs.getBytes() :(
      try {
        // Must use bytes before JDBC connection gets closed
      } finally {
        bytes.close();
      }
    } finally {
      rs.close();
    }
  } finally {
    stmt.close();
  }
} finally {
  connection.close();
}

The astute reader can see (lines also highlighted) how error-prone BLOB handling is with JDBC. The lesser known Connection.createBlob() (introduced in Java 6 / JDBC 4.0) and ResultSet.getBlob(…) is not often used.

File Systems and BLOBs

Another often seen way of handling BLOBs in Java is to store them as files in a file system. But this has some drawbacks, like the tricky to use unique file names, and possible un-tracked tampering of file contents. In some cases, storing them as files may not be permitted when running in a server with a security manager (e.g. Servlet-container like Tomcat with a SecurityManager). The security manager protects the server from trojan servlets, JSPs, JSP beans, tag libraries, or even inadvertent mistakes (e.g. <% System.exit(1); %> in a JSP).

Introducing jBlubble

I find the above handling of BLOBs to be over-complicated. Because of this, I wanted to make BLOB handling easier, and less error-prone. I am also inspired by how Google’s AppEngine for Java was handling BLOBs. Thus, I started looking at the BLOB-related code I have worked with in the past years, and came up with a simple API, I called jBlubble. The main interface is BlobstoreService.

public interface BlobstoreService {
  String createBlob(
      InputStream in,
      String name, String contentType) ...;
  String createBlob(
      BlobstoreWriteCallback callback,
      String name, String contentType) ...;
  BlobInfo getBlobInfo(String blobKey) ...;
  void serveBlob(String blobKey, OutputStream out) ...;
  int[] delete(String... blobKeys);
}

The code is available at my GitHub account. As of this writing, a JDBC-implementation of the API is available. Other implementations are also welcome.

The createBlob methods abstract the JDBC-related code of using java.sql.Blobs, and making sure allocated resources are closed properly. To store BLOBs uploaded to a webapp, the code looks something like:

// Servlet configured to support multipart/form-data
// HttpServletRequest
request.getPart("...").write(fileName);
// Open an input stream with the file (created from uploaded part)
InputStream in = new FileInputStream(fileName);
try {
  ... blobKey = blobstoreService.createBlob(in, ...);
} finally {
  in.close();
}

To serve the previously stored BLOB, the code looks something like:

// HttpServletResponse
BlobInfo blobInfo = blobstoreService.getBlobInfo(blobKey);
response.setContentType(blobInfo.getContentType());
OutputStream out = response.getOutputStream();
blobstoreService.serveBlob(blobKey, out);

In some cases, like when generating reports, the output stream can be stored as a BLOB too, like so:

... JasperPrint print = ...;
... JRExporter exporter = ...; //  new JRPdfExporter();
... blobKey = blobstoreService.createBlob(
    new BlobstoreWriteCallback() {
      @Override
      public long writeToOutputStream(OutputStream out) ... {
        ...
        exporter.setParameter(JRExporterParameter.JASPER_PRINT, print);
        exporter.setParameter(JRExporterParameter.OUTPUT_STREAM, out);
        exporter.exportReport();
        ...
      }
    }, ...);
// generated blobKey can then be
// used to identify the generated report

Since report generation can take its time (especially when the report has several pages), it is a good idea to store the report in a database, rather than directly writing to a servlet output stream. When the report is complete, it can be downloaded (and re-downloaded) anytime.

The key design consideration was to abstract the persistence-related specifics (like JDBC) and use a simpler API. A callback interface was used to support writing to OutputStreams to ensure that it gets closed properly. A higher level method was used to serve the BLOBs directly to an output stream. The BlobInfo contains a timestamp field that can be used to support Last-Modified HTTP header for caching purposes. As of the moment, BLOBs are immutable and cannot be updated. To change, a new BLOB should be created, delete the old one, and reference the new one.

Instead of embedding the BLOB with their related persistent objects (or entities), I find it better to simply reference the BLOB. So, instead of this…

@Entity
public class Person {
  @Id private Long id;
  @Lob byte[] photo;
  ...
}

… I find it better to just reference the BLOB like this.

@Entity
public class Person {
  @Id private Long id;
  String photoId; // retrieved via BlobstoreService
  ...
}

I’ve started replacing existing BLOB handling code with the API we’ve developed. So far, it has simplified things. I hope this will help others too. Now, it’s time for a cold one.

Originally posted at: Blob Handling in Java and JDBC

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