Mongodb 3.2 Java 显示指定字段 条件查询 官方

MongoDB Driver  
Tutorials  
Read Operations

Find Operations

Find operations retrieve documents from a collection. You can specify a filter to select only those documents that match the filter condition.

Prerequisites

The example below requires a 

restaurants

 collection
in the 

test

 database. To create and populate the collection, follow the directions in github.

Include the following import statements:

import com.mongodb.*;
import com.mongodb.MongoClient;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;
import com.mongodb.client.model.Projections;
import com.mongodb.client.model.Filters;
import static com.mongodb.client.model.Filters.*;
import static com.mongodb.client.model.Projections.*;
import com.mongodb.client.model.Sorts;
import java.util.Arrays;
import org.bson.Document;

Include the following code which the examples in the tutorials will use to print the results of the find operations:

Block<Document> printBlock = new Block<Document>() {
@Override
public void apply(final Document document) {
System.out.println(document.toJson());
}
};

Connect to a MongoDB Deployment

Connect to a MongoDB deployment and declare and define a 

MongoDatabase

 instance and a

MongoCollection

 instance

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

27017

 and
define 

database

 to refer to the 

test

 database
and 

collection

 to refer to the 

restaurants

 collection:

MongoClient mongoClient = new MongoClient();
MongoDatabase database = mongoClient.getDatabase("test");
MongoCollection<Document> collection = database.getCollection("restaurants");

For additional information on connecting to MongoDB, see Connect to MongoDB.

Query a Collection

To query the collection, you can use the collection’s 

find()

 method.

You can call the method without any arguments to query all documents in a collection:

collection.find().forEach(printBlock);

Or pass a filter to query for documents that match the filter criteria:

collection.find(eq("name", "456 Cookies Shop"))
.forEach(printBlock);

Query Filters

To query for documents that match certain conditions, pass a filter document to the 

find()

 method.

Empty Filter

To specify an empty filter (i.e. match all documents in a collection), use an empty 

Document

 object.

collection.find(new Document()).forEach(printBlock);

TIP
For the 

find()

 method,
you can also call the method without passing a filter object to match all documents in a collection.

collection.find().forEach(printBlock);

Filters

 Helper

To facilitate the creation of filter documents, the Java driver provides the 

Filters

 class
that provides filter condition helper methods.

Consider the following 

find

 operation which includes a filter 

Document

 which
specifies that:

the 

stars

 field is greater than or equal to
2 and less than 5, AND

the 

categories

 field equals 

"Bakery"

 (or
if 

categories

 is an array, contains the string 

"Bakery"

as
an element):

collection.find(
new Document("stars", new Document("$gte", 2)
.append("$lt", 5))
.append("categories", "Bakery")).forEach(printBlock);

The following example specifies the same filter condition using the 

Filters

 helper
methods:

collection.find(and(gte("stars", 2), lt("stars", 5), eq("categories", "Bakery")))
.forEach(printBlock);

For a list of MongoDB query filter operators, refer to the MongoDB Manual. For the associated 

Filters

helpers,
see 

Filters

.
See also the Query Documents Tutorial for an overview of querying in MongoDB, including specifying filter
conditions on arrays and embedded documents.

FindIterable

The 

find()

 method
returns an instance of the 

FindIterable

 interface.
The interface provides various methods that you can chain to the 

find()

 method to modify
the output or behavior of the query, such as

sort()

 or 

projection()

,
as well as for iterating the results, such as 

iterator()

 and 

forEach()

.

Projections

By default, queries in MongoDB return all fields in matching documents. To specify the fields to return in the matching documents, you can specify a projection
document.

Consider the following 

find

 operation which includes a projection 

Document

 which
specifies that the matching documents return only the 

name

 field, 

stars

 field,
and the 

categories

 field.

collection.find(and(gte("stars", 2), lt("stars", 5), eq("categories", "Bakery")))
.projection(new Document("name", 1)
.append("stars", 1)
.append("categories",1)
.append("_id", 0))
.forEach(printBlock);

To facilitate the creation of projection documents, the Java driver provides the 

Projections

 class.

collection.find(and(gte("stars", 2), lt("stars", 5), eq("categories", "Bakery")))
.projection(fields(include("name", "stars", "categories"), excludeId()))
.forEach(printBlock);

In the projection document, you can also specify a projection expression using a projection
operator

For an example on using the 

Projections.metaTextScore

,
see the Text Search tutorial.

Sorts

To sort documents, pass a sort specification document to the 

FindIterable.sort()

 method.
The Java driver provides 

Sorts

 helpers
to facilitate the sort specification document.

collection.find(and(gte("stars", 2), lt("stars", 5), eq("categories", "Bakery")))
.sort(Sorts.ascending("name"))
.forEach(printBlock);

Sort with Projections

The 

FindIterable

 methods
themselves return 

FindIterable

 objects, and as such, you can append multiple 

FindIterable

 methods
to the 

find()

 method.

collection.find(and(gte("stars", 2), lt("stars", 5), eq("categories", "Bakery")))
.sort(Sorts.ascending("name"))
.projection(fields(include("name", "stars", "categories"), excludeId()))
.forEach(printBlock);

MongoIterable

The 

MongoIterable

 interface
provides helper methods to access the results of an operation:

iterator()

first()

forEach()

map()

into()

Read Preference

For read operations on replica sets or sharded
clusters, applications can configure the read preference at three levels:

In a 

MongoClient()

Via 

MongoClientOptions

:

MongoClientOptions options = MongoClientOptions.builder().readPreference(
ReadPreference.secondary()).build();
MongoClient mongoClient = new MongoClient(Arrays.asList(
new ServerAddress("host1", 27017),
new ServerAddress("host2", 27017)), options);

Via 

MongoClientURI

,
as in the following example:

MongoClient mongoClient = new MongoClient(
new MongoClientURI("mongodb://host1:27017,host2:27017/?readPreference=secondary"));

In a 

MongoDatabase

 via
its 

withReadPreference

 method.

MongoDatabase database = mongoClient.getDatabase("test")
.withReadPreference(ReadPreference.secondary());

In a 

MongoCollection

 via
its 

withReadPreference

 method:

MongoCollection<Document> collection = database.getCollection("restaurants")
.withReadPreference(ReadPreference.secondary());

MongoDatabase

 and 

MongoCollection

 instances
are immutable. Calling 

.withReadPreference()

 on an existing 

MongoDatabase

 or 

MongoCollection

 instance
returns a new instance and does not affect the instance on which the method is called.

For example, in the following, the 

collectionWithReadPref

 instance has the read preference
of primaryPreferred whereas the read preference of the 

collection

 is unaffected.

MongoCollection<Document> collectionWithReadPref =  collection.withReadPreference(ReadPreference.primaryPreferred());

Read Concern

For read operations on replica sets or sharded
clusters, applications can configure the read concern at three levels:

In a 

MongoClient()

Via 

MongoClientURI

,
as in the following example:

MongoClient mongoClient = new MongoClient(
new MongoClientURI("mongodb://host1:27017,host2:27017/?readConcernLevel=majority"));

Via 

MongoClientOptions

,
as in the following example:

MongoClientOptions options = MongoClientOptions.builder().readConcern(ReadConcern.DEFAULT).build();
MongoClient mongoClient = new MongoClient(Arrays.asList(
new ServerAddress("host1", 27017),
new ServerAddress("host1", 27017)), options);

In a 

MongoDatabase

 via
its 

withReadConcern

 method,
as in the following example:

MongoDatabase database = mongoClient.getDatabase("test")
.withReadConcern(ReadConcern.DEFAULT);

In a 

MongoCollection

 via
its 

withReadConcern

 method,
as in the following example:

MongoCollection<Document> collection = database.getCollection("restaurants")
.withReadConcern(ReadConcern.DEFAULT);

MongoDatabase

 and 

MongoCollection

 instances
are immutable. Calling 

.withReadConcern()

 on an existing 

MongoDatabase

 or 

MongoCollection

 instance
returns a new instance and does not affect the instance on which the method is called.

For example, in the following, the 

collWithReadConcern

 instance has majority read concern
whereas the read concern of the 

collection

 is unaffected.

MongoCollection<Document> collWithReadConcern = collection
.withReadConcern(ReadConcern.MAJORITY);

You can build 

MongoClientOptions

, 

MongoDatabase

,
or 

MongoCollection

 to include a combination of read concern, read preference, and write
concern.

For example, the following sets all three at the collection level:

collection = database.getCollection("restaurants")
.withReadPreference(ReadPreference.primary())
.withReadConcern(ReadConcern.MAJORITY)
.withWriteConcern(WriteConcern.MAJORITY);