java将字符串存入GridF并通过id或文件名查询

import static org.bson.codecs.configuration.CodecRegistries.fromProviders;
import static org.bson.codecs.configuration.CodecRegistries.fromRegistries;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.List;

import org.bson.codecs.configuration.CodecProvider;
import org.bson.codecs.configuration.CodecRegistry;
import org.bson.codecs.pojo.PojoCodecProvider;
import org.bson.types.ObjectId;

import com.mongodb.MongoClient;
import com.mongodb.MongoClientOptions;
import com.mongodb.MongoCredential;
import com.mongodb.ServerAddress;
import com.mongodb.client.MongoDatabase;
import com.mongodb.client.gridfs.GridFSBucket;
import com.mongodb.client.gridfs.GridFSBuckets;


public class Test {
    private MongoDatabase database = null;
    public static void main(String[] args) throws Exception {
        Test t=new Test();
        String ID = t.insertMedicalFile("测试一下1.txt", "fjkasdhfkjhdaslfhklahflasdhlhflk");
        String selectMedicalFileByObjectId = t.selectMedicalFileByObjectId("5c91ec501a551431b07eff28");
        String selectMedicalFileByFileName = t.selectMedicalFileByFileName("测试一下1.txt");

    }
    
    /**
     * 向 IMAGE_FILE 中写入文件记录。
     * @param fileName  文件名
     * @param xmlContent   文件内容。
     * @throws Exception
     */
    public String  insertMedicalFile(String fileName , String xmlContent) throws Exception{
        MongoDatabase mds = null;
        mds = getMdbOperator();
        InputStream inputStream = new ByteArrayInputStream(xmlContent.getBytes("UTF-8"));
        //业务逻辑模式
        GridFSBucket gridFSBucket = GridFSBuckets.create(mds,"IMAGE_FILE");
        ObjectId objectId = gridFSBucket.uploadFromStream(fileName, inputStream);
        System.out.println("HUC 生成的 objectId  = " + objectId);
        
        inputStream.close();
        
        return objectId+"" ;
    }
    /**
     * 根据文件id返回文件内容
     * @param objectId
     * @return
     */
    public String selectMedicalFileByObjectId(String objectId){
        String result = "" ;
        try {
            MongoDatabase mds = null;
            mds = getMdbOperator();
            GridFSBucket gridFSBucket = GridFSBuckets.create(mds,"IMAGE_FILE");
            
            ByteArrayOutputStream baos = new ByteArrayOutputStream();   
            gridFSBucket.downloadToStream(new ObjectId(objectId), baos);
            
            result =  baos.toString("UTF-8");
            
        } catch (Exception e) {
            result = "" ; //此处可能会报异常错误，异常错误的原因是 上面 downloadToStream 为空。
            e.printStackTrace();
        }
        return result ; 
    }
    /**
     * 根据文件名返回文件内容
     * @param fileName
     * @return
     */
    public String selectMedicalFileByFileName(String fileName){
        String result = "" ;
        try {
            MongoDatabase mds = null;
            mds = getMdbOperator();
            GridFSBucket gridFSBucket = GridFSBuckets.create(mds,"IMAGE_FILE");
            
            ByteArrayOutputStream baos = new ByteArrayOutputStream();   
            gridFSBucket.downloadToStream(fileName, baos);
            
            result =  baos.toString("UTF-8");
            
        } catch (Exception e) {
            result = "" ; //此处可能会报异常错误，异常错误的原因是 上面 downloadToStream 为空。
            e.printStackTrace();
        }
        return result ; 
    }
    /**
     * 获得链接
     * 
     * @return
     */
    public MongoDatabase getMdbOperator() {
        try {
            MongoClient mongoClient = getMongoClient();
            if (mongoClient != null) {
                CodecProvider pojoCodecProvider = PojoCodecProvider.builder()
                        .automatic(true).build();
                CodecRegistry pojoCodecRegistry = fromRegistries(
                        MongoClient.getDefaultCodecRegistry(),
                        fromProviders(pojoCodecProvider));
                database = mongoClient.getDatabase("HUC")
                        .withCodecRegistry(pojoCodecRegistry);
                return database;
            } else {
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }
    
    

    private static MongoClient getMongoClient() throws Exception {
        MongoClientOptions.Builder build = new MongoClientOptions.Builder();
        // 与目标数据库能够建立的最大connection数量为200,当连接池被用光时，会被阻塞住
        build.connectionsPerHost(200);
        // 如果当前所有的connection都在使用中，则每个connection上可以有100个线程排队等待
        build.threadsAllowedToBlockForConnectionMultiplier(100);
        /*
         * 一个线程访问数据库的时候，在成功获取到一个可用数据库连接之前的最长等待时间为2分钟
         * 这里比较危险，如果超过maxWaitTime都没有获取到这个连接的话，该线程就会抛出Exception
         * 故这里设置的maxWaitTime应该足够大，以免由于排队线程过多造成的数据库访问失败
         */
        build.maxWaitTime(1000 * 60 * 2); // 这个就是Server Selection
                                            // Timeout?
        build.connectTimeout(1000 * 60 * 1); // 与数据库建立连接的timeout设置为1分钟
        build.socketTimeout(10 * 1000); // 发送请求和接收请求的超时时间
        MongoClientOptions myOptions = build.build();
        // 创建链接
        MongoCredential credential = MongoCredential.createCredential(
                "admin", "HUC", "123456".toCharArray());
        List addresses = new ArrayList();
        //mongoDB集群地址
        addresses.add(new ServerAddress("192.168.220.130", 27017));
        addresses.add(new ServerAddress("192.168.220.128", 27017));
        addresses.add(new ServerAddress("192.168.220.129", 27017));
        return new MongoClient(addresses,credential,myOptions);
    }

}