Difference between build.xml and project_name.xml

Question

I am trying to understand the build process of Android for hours altogether now.

I am using IntelliJ Idea for Android (which is wonderful tool). I opened a test project in IntelliJ name "Hello Android". The project has this file name build.xml, code below

<?xml version="1.0" encoding="UTF-8"?>
<project name="HelloAndroid" default="help">

    <!-- The local.properties file is created and updated by the 'android' tool.
         It contains the path to the SDK. It should *NOT* be checked into
         Version Control Systems. -->
    <property file="local.properties" />

    <!-- The ant.properties file can be created by you. It is only edited by the
         'android' tool to add properties to it.
         This is the place to change some Ant specific build properties.
         Here are some properties you may want to change/update:

         source.dir
             The name of the source directory. Default is 'src'.
         out.dir
             The name of the output directory. Default is 'bin'.

         For other overridable properties, look at the beginning of the rules
         files in the SDK, at tools/ant/build.xml

         Properties related to the SDK location or the project target should
         be updated using the 'android' tool with the 'update' action.

         This file is an integral part of the build system for your
         application and should be checked into Version Control Systems.

         -->
    <property file="ant.properties" />

    <!-- if sdk.dir was not set from one of the property file, then
         get it from the ANDROID_HOME env var.
         This must be done before we load project.properties since
         the proguard config can use sdk.dir -->
    <property environment="env" />
    <condition property="sdk.dir" value="${env.ANDROID_HOME}">
        <isset property="env.ANDROID_HOME" />
    </condition>

    <!-- The project.properties file is created and updated by the 'android'
         tool, as well as ADT.

         This contains project specific properties such as project target, and library
         dependencies. Lower level build properties are stored in ant.properties
         (or in .classpath for Eclipse projects).

         This file is an integral part of the build system for your
         application and should be checked into Version Control Systems. -->
    <loadproperties srcFile="project.properties" />

    <!-- quick check on sdk.dir -->
    <fail
            message="sdk.dir is missing. Make sure to generate local.properties using 'android update project' or to inject it through the ANDROID_HOME environment variable."
            unless="sdk.dir"
    />

    <!--
        Import per project custom build rules if present at the root of the project.
        This is the place to put custom intermediary targets such as:
            -pre-build
            -pre-compile
            -post-compile (This is typically used for code obfuscation.
                           Compiled code location: ${out.classes.absolute.dir}
                           If this is not done in place, override ${out.dex.input.absolute.dir})
            -post-package
            -post-build
            -pre-clean
    -->
    <import file="custom_rules.xml" optional="true" />

    <!-- Import the actual build file.

         To customize existing targets, there are two options:
         - Customize only one target:
             - copy/paste the target into this file, *before* the
               <import> task.
             - customize it to your needs.
         - Customize the whole content of build.xml
             - copy/paste the content of the rules files (minus the top node)
               into this file, replacing the <import> task.
             - customize to your needs.

         ***********************
         ****** IMPORTANT ******
         ***********************
         In all cases you must update the value of version-tag below to read 'custom' instead of an integer,
         in order to avoid having your file be overridden by tools such as "android update project"
    -->
    <!-- version-tag: 1 -->
    <import file="${sdk.dir}/tools/ant/build.xml" />

</project>

Then I did the following in IntelliJ Idea Build -> Generate Ant Build and selected the following options

1. Generate multiple-file ant build
2. Output file name: helloandroid
3. Enable UI forms compilation
4. Use JSDK definitions from project files

This resulted in creation of three additional files

1. helloandroid.properties
2. helloandroid.xml
3. module_helloandroid.xml

helloandroid.properties

path.variable.maven_repository=C\:\\Users\\abc\\.m2\\repository
jdk.home.android_4.1_platform=C\:/Program Files (x86)/Android/android-sdk

helloandroid.xml

<?xml version="1.0" encoding="UTF-8"?>
<project name="HelloAndroid" default="help">

    <!-- The local.properties file is created and updated by the 'android' tool.
         It contains the path to the SDK. It should *NOT* be checked into
         Version Control Systems. -->
    <property file="local.properties" />

    <!-- The ant.properties file can be created by you. It is only edited by the
         'android' tool to add properties to it.
         This is the place to change some Ant specific build properties.
         Here are some properties you may want to change/update:

         source.dir
             The name of the source directory. Default is 'src'.
         out.dir
             The name of the output directory. Default is 'bin'.

         For other overridable properties, look at the beginning of the rules
         files in the SDK, at tools/ant/build.xml

         Properties related to the SDK location or the project target should
         be updated using the 'android' tool with the 'update' action.

         This file is an integral part of the build system for your
         application and should be checked into Version Control Systems.

         -->
    <property file="ant.properties" />

    <!-- if sdk.dir was not set from one of the property file, then
         get it from the ANDROID_HOME env var.
         This must be done before we load project.properties since
         the proguard config can use sdk.dir -->
    <property environment="env" />
    <condition property="sdk.dir" value="${env.ANDROID_HOME}">
        <isset property="env.ANDROID_HOME" />
    </condition>

    <!-- The project.properties file is created and updated by the 'android'
         tool, as well as ADT.

         This contains project specific properties such as project target, and library
         dependencies. Lower level build properties are stored in ant.properties
         (or in .classpath for Eclipse projects).

         This file is an integral part of the build system for your
         application and should be checked into Version Control Systems. -->
    <loadproperties srcFile="project.properties" />

    <!-- quick check on sdk.dir -->
    <fail
            message="sdk.dir is missing. Make sure to generate local.properties using 'android update project' or to inject it through the ANDROID_HOME environment variable."
            unless="sdk.dir"
    />

    <!--
        Import per project custom build rules if present at the root of the project.
        This is the place to put custom intermediary targets such as:
            -pre-build
            -pre-compile
            -post-compile (This is typically used for code obfuscation.
                           Compiled code location: ${out.classes.absolute.dir}
                           If this is not done in place, override ${out.dex.input.absolute.dir})
            -post-package
            -post-build
            -pre-clean
    -->
    <import file="custom_rules.xml" optional="true" />

    <!-- Import the actual build file.

         To customize existing targets, there are two options:
         - Customize only one target:
             - copy/paste the target into this file, *before* the
               <import> task.
             - customize it to your needs.
         - Customize the whole content of build.xml
             - copy/paste the content of the rules files (minus the top node)
               into this file, replacing the <import> task.
             - customize to your needs.

         ***********************
         ****** IMPORTANT ******
         ***********************
         In all cases you must update the value of version-tag below to read 'custom' instead of an integer,
         in order to avoid having your file be overridden by tools such as "android update project"
    -->
    <!-- version-tag: 1 -->
    <import file="${sdk.dir}/tools/ant/build.xml" />

</project>

module_helloandroid.xml

<copy todir="${helloandroid.output.dir}">
  <fileset dir="${module.helloandroid.basedir}/src">
    <patternset refid="compiler.resources"/>
    <type type="file"/>
    <patternset refid="excluded.from.compilation.helloandroid"/>
  </fileset>
  <fileset dir="${module.helloandroid.basedir}/gen">
    <patternset refid="compiler.resources"/>
    <type type="file"/>
    <patternset refid="excluded.from.compilation.helloandroid"/>
  </fileset>
</copy>

My Question

1. Why we have different build.xml and helloandroid.xml and other two files?
2. Can these files be used in the build process?
3. Can we edit these files to add more Ant build script?
4. Why build.xml is showing an error at env.ANDROID_HOME?
5. What helloandroid.xml is showing an error at jdk.home.1.6?

Answer 1

1. Build files can include other build files, this is a normal practice. your project's helloandroid.xml is exactly identical to the build.xml file, you can modify whichever one you like and discard the other. The build.xml file is the preferred convention for Ant. Your properties file is just for setting properties, and the module file doesn't look to be called, but probably exists so that you can include it and do the file copying as it specifies.

2. Yes, just include them. The helloandroid.properties file can be used instead of the call to the generic project.properties file, or you can leave it as is.

3. Yes, please edit them, though they should be sufficient for an easy start.

4. It looks like your ANDROID_HOME environment variable has not been set. You can set it through IntelliJ, but I would recommend doing it at your OS level.

5. It looks like your JAVA_HOME environment variable has not been set. You can set it through IntelliJ, but I would recommend doing it at your OS level.