How does maven find Java compiler?

Question

I am new to learn maven by following this example given by sonatype's book.This is a simplest pom.xml file (works fine). But my question is, how does maven (mvn install) find Java compiler (javac) installed on my machine? This POM file doesn't specify JDK in any way. Thanks for explanation.

EDIT: As a comparison, I am following next example in the book and this time, I received a compilation error because it can't find javax.My java file is in, as suggested in the book: src/main/java/org/sonatype/mavenbook/web/SimpleServlet.java

Now as I run mvn compile, and i got error message below. Why now maven can't find the compiler:

ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.1:compile (default-compile) on project simple-webapp: Compilation failure: Compilation failure:
[ERROR] /home/abigail/study/simple-webapp/src/main/java/org/sonatype/mavenbook/web/SimpleServlet.java:[4,1] package javax.servlet does not exist

POM.xml

<project xmlns="http://maven.apache.org/POM/4.0.0"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
                         http://maven.apache.org/maven-    v4_0_0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>org.sonatype.mavenbook.simpleweb</groupId>
<artifactId>simple-webapp</artifactId>
<packaging>war</packaging>
<version>1.0-SNAPSHOT</version>
<name>simple-webapp Maven Webapp</name>
<url>http://maven.apache.org</url>
<dependencies>
    <dependency>
        <groupId>junit</groupId>
        <artifactId>junit</artifactId>
        <version>3.8.1</version>
        <scope>test</scope>
    </dependency>
</dependencies>
<build>
    <finalName>simple-webapp</finalName>
    <plugins>
        <plugin>
            <artifactId>maven-compiler-plugin</artifactId>
            <configuration>
                <source>1.7</source>
                <target>1.7</target>
            </configuration>
        </plugin>
    </plugins>
</build>
</project>

Answer 1

If you go to this path, say with maven version 3.2.3

apache-maven-3.2.3-bin\apache-maven-3.2.3\bin

opne the mvn.bat, you can see below

@REM Maven2 Start Up Batch script

@REM Required ENV vars:

@REM JAVA_HOME - location of a JDK home dir

Clearly it picks JAVA_HOME, from your environment variables. Below code snippet from the file.

@REM ==== START VALIDATION ====
if not "%JAVA_HOME%" == "" goto OkJHome
:OkJHome
if exist "%JAVA_HOME%\bin\java.exe" goto chkMHome
:chkMHome
if not "%M2_HOME%"=="" goto valMHome

for issue, javax.servlet

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>3.0.1</version>
    <scope>provided</scope>
</dependency>

or whichever available

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>servlet-api</artifactId>
    <version>2.5</version>
    <scope>provided</scope>
</dependency>

Answer 2

Maven uses the environment variable JAVA_HOME to find the JDK.