1. 第一步, 生成查询对象
   在一个jpa项目中加入如下依赖和配置:

<dependency>
    <groupId>com.querydsl</groupId>
    <artifactId>querydsl-jpa</artifactId>
</dependency>
<dependency>
    <groupId>com.querydsl</groupId>
    <artifactId>querydsl-apt</artifactId>
    <scope>provided</scope>
</dependency>

<build>
    <plugins>
        <plugin>
            <groupId>com.mysema.maven</groupId>
            <artifactId>apt-maven-plugin</artifactId>
            <version>1.1.3</version>
            <executions>
                <execution>
                    <goals>
                        <goal>process</goal>
                    </goals>
                    <configuration>
                        <outputDirectory>target/generated-sources/java</outputDirectory>
                        <processor>com.querydsl.apt.jpa.JPAAnnotationProcessor</processor>
                    </configuration>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

如果报找不到Inject类还需要加入:

<dependency>
    <groupId>javax.inject</groupId>
    <artifactId>javax.inject</artifactId>
    <version>1</version>
</dependency>

然后去命令行运行mvn clean compile (IDEA的compile好像不行), 插件会扫描@Entity对象并且按名称生成查询对象, 例如@Entity class Country会生成class QCountry类, 代码在指定的outputDirectory里

将生成的查询对象移动到项目里

repository继承QueryDslPredicateExecutor<T>

public interface CountryRepository extends JpaRepository<Country, String>, QueryDslPredicateExecutor<Country> {
}

下边是一个条件查询的例子, 使用了Pageable

private Page<Country> searchByContinent(String continent, Pageable pageable) {
    BooleanBuilder booleanBuilder = new BooleanBuilder();
    if (hasLength(continent)) {
        booleanBuilder.and(QCountry.country.continent.eq(continent));
    }

    return countryRepository.findAll(booleanBuilder.getValue(), pageable);
}