There are several options to specify.
Steps: Right on project in project explorer Go to Run-> Run Configuration -> Click Maven Build -> Click on your build config/or create a new config. You will see the window as the given snapshot below, click on JRE tab there.
You see you have 3 options 1) Workspace Default JRE 2)Execution Environment 3)Alternate JRE 1) Workspace Default JRE is set from 'Window' menu on the top -> Preferences -> Java -> Installed JREs -Here you can add your jdk 2) Execution Environment jdk can be set in pom.xml
<build> <plugins> <plugin> <artifactId>maven-compiler-plugin</artifactId> <version>3.1</version> <configuration> <fork>true</fork> <executable>C:\Program Files\Java\jdk1.7.0_45\bin\javac.exe</executable> </configuration> </plugin> </plugins> 3) Alternate JRE can be used to select a jdk from your directory

There are several options to specify.
Steps: Right on project in project explorer Go to Run-> Run Configuration -> Click Maven Build -> Click on your build config/or create a new config. You will see the window as the given snapshot below, click on JRE tab there.
You see you have 3 options 1) Workspace Default JRE 2)Execution Environment 3)Alternate JRE [![enter image description here][1]][1] 1) Workspace Default JRE is set from 'Window' menu on the top -> Preferences -> Java -> Installed JREs -Here you can add your jdk [![enter image description here][2]][2] 2) Execution Environment jdk can be set in pom.xml as mentioned by @ksnortum
<build> <plugins> <plugin> <artifactId>maven-compiler-plugin</artifactId> <version>3.1</version> <configuration> <fork>true</fork> <executable>C:\Program Files\Java\jdk1.7.0_45\bin\javac.exe</executable> </configuration> </plugin> </plugins> </build>
3) Alternate JRE can be used to select a jdk from your directory

[1]: https://i.stack.imgur.com/GLD8o.png [2]: https://i.stack.imgur.com/XsW0U.png