# 访问数据库
具体访问数据库前，引入如下相关类。
```
import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoClient;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;
import static com.mongodb.client.model.Filters.*;
import com.mongodb.client.model.CreateCollectionOptions;
import com.mongodb.client.model.ValidationOptions;
```
#### 访问DataBase
当已经有一个初始化好的MongoClient实例后，通过如下方式来访问一个database，示例如下：
```
MongoDatabase database = mongoClient.getDatabase("test");
```
#### 访问集合
当获取一个MongoDatabase实例后，可以通过如下命令来得到要获取的集合：
```
MongoCollection<Document> coll = database.getCollection("testCollection");
```
#### 显示的创建一个集合
也可以通过 createCollection()方法显示的创建一个集合，并在创建时候指定该集合的属性。
```
database.createCollection("testCollection", new CreateCollectionOptions()..sizeInBytes(200000))
```
#### 插入数据
```
Document doc0 = new Document("name", "zhangsan")       
               .append("age", 3)
               .append("gender", "male");
Document doc1 = new Document("name", "LiSi")
               .append("age", 2)
               .append("gender", "female");
Document doc2 = new Document("name", "wangmazi")
               .append("age", 5)
               .append("gender", "male");
List<Document> documents = new ArrayList<Document>();
documents.add(doc1);
documents.add(doc2);
collection.insertMany(documents);
```
#### 删除数据
```
collection.deleteOne(eq("_id", new ObjectId("00000001")));
```
#### 删除表
```
MongoCollection<Document> collection = database.getCollection("test");
collection.drop()
```
#### 读数据
```
MongoCollection<Document> collection = database.getCollection("contacts");
MongoCursor<String> cursor = collection.find(); 
while (cursor.hasNext())  {
   Object result = cursor.next(); 
}
```
#### 带过滤条件的查询
```
MongoCollection<Document> collection = database.getCollection("test");
MongoCursor<String> cursor = collection.find(
    new Document("name","zhangsan")
          .append("age: 5));
while (cursor.hasNext())  {
   Object result = cursor.next(); 
}
```
#### 运行命令
执行buildInfo和collStats
```
MongoClient mongoClient = MongoClients.create();
MongoDatabase database = mongoClient.getDatabase("test");
Document buildInfoResults = database.runCommand(new Document("buildInfo", 1));
System.out.println(buildInfoResults.toJson());
Document collStatsResults = database.runCommand(new Document("collStats", "restaurants"));
System.out.println(collStatsResults.toJson());
```
#### 创建建索引
```
MongoClient mongoClient = MongoClients.create();
MongoDatabase database = mongoClient.getDatabase("test");
MongoCollection<Document> collection = database.getCollection("test");
collection.createIndex(Indexes.ascending("age"));
```