官网关于GridFS操作的说明文档_gridfsuploadoptions

GridFS

GridFS is a specification for storing and retrieving files that exceed the BSON document size limit of 16MB. Instead of storing a file in a single document, GridFS divides a file into parts, or chunks, and stores each of those chunks as a separate document.

When you query a GridFS store for a file, the Java driver will reassemble the chunks as needed.

The following code snippets come from the GridFSTour.java example code that can be found with the driver source on github.

Prerequisites

Include the following import statements:

import com.mongodb.Block;
import com.mongodb.MongoClient;
import com.mongodb.client.MongoDatabase;
import com.mongodb.client.gridfs.*;
import com.mongodb.client.gridfs.model.*;
import org.bson.Document;
import org.bson.types.ObjectId;
import java.io.*;
import java.nio.file.Files;
import java.nio.charset.StandardCharsets;
import static com.mongodb.client.model.Filters.eq;
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Connect to a MongoDB Deployment

Connect to a MongoDB deployment and declare and define a MongoDatabase instance.

For example, include the following code to connect to a standalone MongoDB deployment running on localhost on port 27017:

MongoClient mongoClient = new MongoClient();
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For additional information on connecting to MongoDB, see Connect to MongoDB.

Create a GridFS Bucket

GridFS stores files in two collections: a chunks collection stores the file chunks, and a files collection stores file metadata. The two collections are in a common bucket and the collection names are prefixed with the bucket name.

The Java driver provides the GridFSBuckets.create() method to create the GridFSBucket.

MongoDatabase myDatabase = mongoClient.getDatabase("mydb");

// Create a gridFSBucket using the default bucket name "fs"
GridFSBucket gridFSBucket = GridFSBuckets.create(myDatabase);
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You can specify a bucket name to GridFSBuckets.create() method.

// Create a gridFSBucket with a custom bucket name "files"
GridFSBucket gridFSFilesBucket = GridFSBuckets.create(myDatabase, "files");
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NOTE
GridFS will automatically create indexes on the files and chunks collections on first upload of data into the GridFS bucket.

Upload to GridFS

To upload data into GridFS, you can upload from an InputStream or write data to a GridFSUploadStream.

UploadFromStream

The GridFSBucket.uploadFromStream method reads the contents of an InputStream and saves it to the GridFSBucket.

You can use the GridFSUploadOptions to configure the chunk size or include additional metadata.

The following example uploads an InputStream into GridFSBucket:

// Get the input stream

try {
    InputStream streamToUploadFrom = new FileInputStream(new File("/tmp/mongodb-tutorial.pdf"));
    // Create some custom options
    GridFSUploadOptions options = new GridFSUploadOptions()
                                        .chunkSizeBytes(358400)
                                        .metadata(new Document("type", "presentation"));

    ObjectId fileId = gridFSBucket.uploadFromStream("mongodb-tutorial", streamToUploadFrom, options);
} catch (FileNotFoundException e){
   // handle exception
}
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OpenUploadStream

You can write data to a GridFSUploadStream which extends OutputStream. The GridFSBucket.openUploadStream method returns a GridFSUploadStream.

The GridFSUploadStream buffers data until it reaches the chunkSizeBytes and then inserts the chunk into the chunks collection. When the GridFSUploadStream is closed, the final chunk is written and the file metadata is inserted into the files collection.

The following example uploads into a GridFSBucket via the returned GridFSUploadStream:

try {
    GridFSUploadOptions options = new GridFSUploadOptions()
                       .chunkSizeBytes(358400)
                       .metadata(new Document("type", "presentation"));

    GridFSUploadStream uploadStream = gridFSFilesBucket.openUploadStream("mongodb-tutorial-2", options);
    byte[] data = Files.readAllBytes(new File("/tmp/MongoDB-manual-master.pdf").toPath());

    uploadStream.write(data);
    uploadStream.close();
    System.out.println("The fileId of the uploaded file is: " + uploadStream.getObjectId().toHexString());

} catch(IOException e){
    // handle exception
}
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Find Files Stored in GridFS

To find the files stored in the GridFSBucket use the find method.

The following example prints out the filename of each file stored:

gridFSBucket.find().forEach(
  new Block<GridFSFile>() {
    public void apply(final GridFSFile gridFSFile) {
        System.out.println(gridFSFile.getFilename());
    }
});
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You can also provide a custom filter to limit the results returned. The following example prints out the filenames of all files with a “image/png” value set as the contentType in the user defined metadata document:

gridFSBucket.find(eq("metadata.contentType", "image/png")).forEach(
  new Block<GridFSFile>() {
      public void apply(final GridFSFile gridFSFile) {
          System.out.println(gridFSFile.getFilename());
      }
  });
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Download from GridFS

There are various ways to download data from GridFS.

DownloadToStream

The downloadToStream method reads the contents from MongoDB and writes the data directly to the provided OutputStream.

To download a file by its file _id, pass the _id to the method. The following example downloads a file by its file _id into the provided OutputStream:

ObjectId fileId; //The id of a file uploaded to GridFS, initialize to valid file id 

try {
    FileOutputStream streamToDownloadTo = new FileOutputStream("/tmp/mongodb-tutorial.pdf");
    gridFSBucket.downloadToStream(fileId, streamToDownloadTo);
    streamToDownloadTo.close();
    System.out.println(streamToDownloadTo.toString());
} catch (IOException e) {
    // handle exception
}
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If you don’t know the _id of the file but know the filename, then you can pass the filename to the downloadToStream method. By default, it will download the latest version of the file. Use the GridFSDownloadOptions to configure which version to download.

The following example downloads the original version of the file named “mongodb-tutorial” into the OutputStream:

try {
    FileOutputStream streamToDownloadTo = new FileOutputStream("/tmp/mongodb-tutorial.pdf");
    GridFSDownloadOptions downloadOptions = new GridFSDownloadOptions().revision(0);
    gridFSBucket.downloadToStream("mongodb-tutorial", streamToDownloadTo, downloadOptions);
    streamToDownloadTo.close();
} catch (IOException e) {
   // handle exception
}
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OpenDownloadStream

The openDownloadStream method returns a GridFSDownloadStream which extends InputStream.

The following example reads from the GridFSBucket via the returned InputStream:

ObjectId fileId; //The id of a file uploaded to GridFS, initialize to valid file id 

GridFSDownloadStream downloadStream = gridFSBucket.openDownloadStream(fileId);
int fileLength = (int) downloadStream.getGridFSFile().getLength();
byte[] bytesToWriteTo = new byte[fileLength];
downloadStream.read(bytesToWriteTo);
downloadStream.close();

System.out.println(new String(bytesToWriteTo, StandardCharsets.UTF_8));
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You can also pass the filename to the openDownloadStream method. By default it will download the latest version of the file. Use the GridFSDownloadOptions to configure which version to download.

The following example downloads the latest version of the file named “sampleData” into the OutputStream:

GridFSDownloadStream downloadStream = gridFSBucket.openDownloadStream("sampleData");
int fileLength = (int) downloadStream.getGridFSFile().getLength();
byte[] bytesToWriteTo = new byte[fileLength];
downloadStream.read(bytesToWriteTo);
downloadStream.close();

System.out.println(new String(bytesToWriteTo, StandardCharsets.UTF_8));
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Rename files

If you should need to rename a file, then use the rename method.

The following example renames a file to “mongodbTutorial”:

ObjectId fileId; //ObjectId of a file uploaded to GridFS

gridFSBucket.rename(fileId, "mongodbTutorial");
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NOTE
The rename method requires an ObjectId rather than a filename to ensure the correct file is renamed.

To rename multiple revisions of the same filename, first retrieve the full list of files. Then for every file that should be renamed then execute rename with the corresponding _id.

Delete files

To delete a file from the GridFSBucket use the delete method.

The following example deletes a file from the GridFSBucket:

ObjectId fileId; //ObjectId of a file uploaded to GridFS

gridFSBucket.delete(fileId);
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