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Gradle is a build manager based upon an Ant-like task dependency graph expressed in a more human-friendly notation, with a Maven-like ability to express standard project layouts and build conventions.
Gradle devised by GradleWare, founded by Hans Dockter, released in 2012
Has become the standard build tool for Android
Tries to strike a middle ground between Ant and Maven
Keep:
Leave
“XML was an easy choice for a build tool a decade ago, when it was a new technology, developers were enthusiastic about it, and no one yet knew the pain of reading it in large quantities. It seemed to be human-readable, and it was very easy to write code to parse it. However, a decade of experience has shown that large and complex XML files are only easy for machines to read, not for humans. Also, XML’s strictly hierarchical structure limits the expressiveness of the format. It’s easy to show nesting relationships in XML, but it’s hard to express program flow and data access the way most common programming language idioms express them. Ultimately, XML is the wrong format for a build file.”
Tim Berglund, Learning and Testing with Gradle
Keep:
Dependency management
Archetypes
Leave
XML as a build language.
Inflexibility
Inability to express simple control flow
Suppose that you are building your project with Gradle.
If you set up your project with the Gradle Wrapper, you get a simple script named gradlew.
gradlew checks to see if the system on which it is running has Gradle installed.
$USER_HOME/.gradlegradle (old installation or new), passing any parameters you supplied to the gradlew command.This works nicely for projects that distribute their source code via any of the version control systems that we will be discussing later.
A gradle project is controlled by two build files
settings.gradle in the project root directory gives project-wide info
build.gradle, in each subproject directory, desctibes the tasks for that subprojectI’ll work through a progressive series of builds for this project.
gradle build
or, if you are using the gradlew wrapper (and you should)
./gradlew build
(possibly modified with the appropriate path to your copy of the wrapper), will compile the production code, compile the unit test code, and will run the unit tests and prepare an HTML report.
build is the most common. To see the list of possible targets (tasks), give the command
./gradlew tasks --all
Using the Java plugin, a basic build without unit tests is easy:
settings.gradle
// A simple Java project
This file needs to exist, but can be empty.
build.gradle
plugins {
id 'java' ➀
}
repositories { ➁
mavenCentral()
}
➀ This loads and runs the Java plugin.
As long as our source code is arranged in the Apache/Android directory style, it will be handled correctly.
➁ The repositories section loads from mavenCentral (the default). Even this could probably be omitted in this build.
This is all you need to compile Java code stores in src/main/java.
plugins {
id 'java'
}
java {
sourceCompatibility = JavaVersion.toVersion(11) ➀
targetCompatibility = JavaVersion.toVersion(11)
}
repositories { ➁
mavenCentral()
}
dependencies { ➂
implementation 'com.opencsv:opencsv:5.1'
implementation 'commons-io:commons-io:2.6'
}
➀ Most open-source libraries in public repositories are compiled with Java 11, because Java 12 and upwards introduced a “modules” system incompatible with older code. So, unless you really need newer Java features, it makes sense to compile your code with Java 11 compatibility when working with open-source libraries.
➁ The repositories area identifies the locations from which third party libraries will be downloaded.
➂ The dependencies list identifies the libraries that we want to use. We’ll look at this in more detail in a later lesson.
If you have unit tests, we need to add a little more info.
build.gradle
plugins {
id 'java'
}
java {
sourceCompatibility = JavaVersion.toVersion(11)
targetCompatibility = JavaVersion.toVersion(11)
}
repositories {
mavenCentral()
}
dependencies {
testImplementation 'org.junit.jupiter:junit-jupiter:5.7.0' ➀
testImplementation 'org.hamcrest:hamcrest-library:2.2'
}
test { ➁
ignoreFailures = true ➂
useJUnitPlatform()
}
The two sections at the end establish that
src/test directory contains our JUnit tests.
A multi-project build in gradle is kept in a directory tree.
The top-most directory is the master project.
It contains the settings.gradle file.
Each subproject directory contains its own build.gradle file.
Any multi-project build in Gradle uses the settings.gradle file (usually in the common root containing the subproject directories) to identify the subprojects:
rootProject.name = 'manhattan'
include "application", "geomlib"
In this case, we have two subprojects. One provides the executable, the other a library of ADTs, with unit tests, constituting the bulk of the code design.
There is no need for build.gradle file at this level, although it would be legal to provide one if you have project-level steps to be carried out.
Of the two subprojects, the geomlib subproject is probably most interesting because it does the most. It compiles code into a library, then compiles and runs unit tests on that library code. By contrast, the application subproject only compiles code.
Here is the build.gradle file for the lib subproject:
plugins {
id 'cpp-library'
id 'cpp-unit-test'
}
unitTest {
binaries.whenElementKnown(CppTestExecutable) { binary ->
if (binary.targetMachine.operatingSystemFamily.linux) {
binary.linkTask.get().linkerArgs.add('-pthread')
}
}
}
This listing starts off with two plugins, one for C++ compilation (into a reusable library) and the other for unit testing.
The unit test plugin can work with a variety of frameworks. Eventually, you should be able to load these frameworks as dependencies, much as you do in Java projects.
In this case, I am using CppUnitLite because it can be easily dropped into the directory with the unit tests.
Like most unit test frameworks, this one runs the tests in a separate thread (process). The unittest configuration here adds the required pthread library when compiling under Linux.
The application subproject is simpler, because it only has one job to do – create an executable. This project depends on the geomlib subproject. The library must have been constructed before the code in the application subproject can be compiled.
Here it is:
plugins {
id 'cpp-application'
}
dependencies {
implementation project(':geomlib')
}
cpp-application plugin and:geomlib subproject
This dependency guarantees that the geomlib library will be constructed before this application is built and that it will be automatically included into the application compilation and link steps.
You can find this entire project, with the Gradle files, here.
The gradle build files are Groovy scripts, with Java-like syntax. Many of the more common Java API packages are imported automatically.
task upper {
doLast {
String myName = "Steven Zeil";
println myName;
println myName.toUpperCase();
}
}
If this is placed in build.gradle, then we can run it:
$ gradle upper
:upper
Steven Zeil
STEVEN ZEIL
BUILD SUCCESSFUL
Total time: 1.747 secs
$
The basic elements of a Gradle build file are tasks.
Gradle tasks can correspond Ant targets.
A Gradle task can perform multiple actions.
task upper {
doLast {
String myName = "Steven Zeil";
println myName;
println myName.toUpperCase();
}
}
Gradle tasks can correspond individual Ant tasks.
A Gradle task can perform multiple actions.
task copyResources (type: Copy) {
from(file('src/main/resources'))
into(file('target/classes'))
}
In Ant, we would have used a <copy> task within a larger Ant target for this purpose.
Before looking at the components of a task, we need to understand a bit about how Gradle works.
A Gradle run occurs in four specific phases.
Initialization takes care of things that affect how Gradle itself will run. The most visible activity during this phase is loading Gradle plugins that add new behaviors.
Configuration involves collecting the build’s tasks, setting the properties of those tasks, and then deciding which tasks need to be executed and the order in which that will be done.
Execution is when the tasks that need to be executed get run.
Finalization covers any needed cleanup before the Gradle run ends.
A Gradle task can be declared as easily as:
task myTask
A Groovy function with parameters, e.g.,
void foo (int x, int y, int z) { ... }
can be called using positional arguments:
foo (12, 14, 0)
or by using named arguments:
foo (z: 0, x: 12, y: 14)
One advantage of the latter form is that you don’t have to know the order in which the parameters appeared in the function declaration. The named form is even more useful when all or most of the parameters have default values and, in your call, you only want to supply the value to one or two parameters for which you don’t like the defaults.
Some tasks may need parameters:
task copyResources (type: Copy)
You can add code to be run at configuration time by putting it in { } brackets just after the task name:
task copyResources (type: Copy)
copyResources {
description = 'Copy resources into a directory from which they will be added to the Jar' ➀
group = 'setup' ➁
from(file('src/main/resources')) ➂
into(file('target/classes'))
}
description supplies a documention string that will be displayed when someone runs gradle tasks to get a list of available tasks for ap roject.group is also documentation. Tasks in the same group are listed together.from and into parameters are the actual parameters for the Copy task type.You can combine a configuration with the task declaration:
task copyResources (type: Copy) {
description = 'Copy resources into a directory from which they will be added to the Jar'
group = 'setup'
from(file('src/main/resources'))
into(file('target/classes'))
}
For example, the Copy type copies files at execution time
task copyResources (type: Copy) {
from(file('src/main/resources'))
into(file('target/classes'))
}
The from and into calls are performed at configuration time.
The Copytype actually copies the files at execution time.
gradle will check to see, at configuration time, if the files already exist at the destination and appear to be no olderthan the ones at the source. If so, the copyResources task will be skipped at execution time.{ }, using the doLast operation.
task copyResources (type: Copy) {
from(file('src/main/resources'))
into(file('target/classes'))
doLast {
println 'Copy has been done.'
}
}
or you can use that operation to add the code to an already-declared task object.
task copyResources (type: Copy) {
from(file('src/main/resources'))
into(file('target/classes'))
}
⋮
copyResources.doLast {
println 'Copy has been done.'
}
task setupFormat
task setupGraphics
task setupSourceCode
task generatePDFs (dependsOn: 'setup')
generatePDFs.doLast { ➀
println 'in task generatePDFs'
}
task setup (dependsOn: ['setupFormat',
'setupGraphics',
'setupSourceCode'])
setup.doLast { ➁
println 'in task setup'
}
task deploy (dependsOn: generatePDFs)
deploy.doLast {
println 'in task deploy'
}
➀ : This line shows a dependency. Note that the task on which we are depending has not been declared yet.
➁ : The [a, b, ...] notation introduces a Groovy array value.
Running this gives:
$ gradle deploy
:setupFormat UP-TO-DATE
:setupGraphics UP-TO-DATE
:setupSourceCode UP-TO-DATE
:setup
in task setup
:generatePDFs
in task generatePDFs
:deploy
in task deploy
Gradle will attempt to determine whether tasks actually need to be run by examining their inputs and outputs, checking to see if the outputs exist already and are more up-to-date than the inputs.
“Built-in” task types like Copy will tell Gradle about their inputs and outputs based upon the other parameters (from and into). When we write our own tasks, it’s advisable to declare the task input and outputs explicitly. If we don’t Gradle will have to run the task on every build where the dependsOn relations suggest that it is relevant.
For example, suppose that I have a Java project that includes a program in class CS350.GenerateTable that converts .csv files into HTML tables, and that I want to use it, as part of my build, to create a table from src/main/data/table1.csv. The basic task will look like this:
task generateTable1 (dependsOn: 'jar') { ➀
doLast {
exec { ➁
commandLine = ['java', '-cp', 'build/libs/myProject.jar', 'CS350.GenerateTable',
'src/main/data/table1.csv', 'build/data/table1.html']
}
}
}
jar so that the Java source code will have been compiled and packaged into a jar file before we try to execute it.exec is a Gradle function that executes commands as if typed at the command line of a shell. In this case, we are running our compiled Java program.
Besides the commandLine parameter, exec can also be supplied a workingDir parmaeter for the directory from which to run the command-line, but that defaults to ‘.’.
Now, this task would work, but if we had other tasks that dpeend upon it, Gradle would run generateTable1 every time, because it can’t possibly figure out from our command line what are the inputs and what are the outputs.
We can supply that information as part of the task configuration:
task generateTable1 (dependsOn: 'jar') {
inputs.file('build/libs/myProject.jar') ➀
inputs.file('src/main/data/table1.csv')
outputs.file('build/data/table1.html')
doLast {
exec {
commandLine = ['java', '-cp', 'build/libs/myProject.jar', 'CS350.GenerateTable',
'src/main/data/table1.csv', 'build/data/table1.html']
}
}
}
inputs and outputs specifications cna be repeated as necessary to describe all inputs.
Common variants on the .file calls shown here are
.dir(path) to indicate that the entire contents of a directory are inputs/outputs..property(name), for inputs only, to indicate that a named property (e.g., commandLine above) is actually an input so that the task should be rerun when the value of that property changes..upToDateWhen({expression}), for outputs only, allows you to specify a condition (boolean expression) to indicate when the task is already up-to-date.doLast (and doFirst) add actions to a task.
If we add the following to the previous script:
setup.doLast {
println 'still in task setup'
}
$ gradle deploy
:setupFormat UP-TO-DATE
:setupGraphics UP-TO-DATE
:setupSourceCode UP-TO-DATE
:setup
in task setup
still in task setup
:generatePDFs
in task generatePDFs
:deploy
in task deploy
At its heart, a build file is a collection of tasks and declarations.
A task is a Groovy function. It has a name, a body, and, optionally,
The body of a task can contain multiple declarations and commands.
Gradle tasks are equivalent to Ant “targets”.
task playWithFiles {
doLast {
def files = file('src/main/data').listFiles().sort() ➀
files.each { File file -> ➁
if (file.isFile()) { ➂
def size = file.length() ➃
println "** $file.name has length " + size ➄
}
}
}
}
➀ There’s several things going on here. Let’s take it piece by piece.
def declares a variable. In this case, the variable is named files.
Like most scripting languages, Groovy (and therefore Gradle) is loosely (dynamically) typed, so we generally do not bother declaring variables by giving their type, though that’s certainly possible.
file(...) returns a value of type File. In fact, this is a good old fashioned Java java.io.File, so we can look to the Java documentation to see what can be done with it.
One of the things we can do with it is to call listFiles(), which treats the File on the left of the ‘.’ as a directory and produces an array of File representing all the files in that directory.
And, knowing that listFiles() produces and array of files, it’s pretty obvious what .sort() would do.
We conclude that the variable files will hold a sorted list of all the files in directory src/main/data.
➁ The .each function is a Groovy function on arrays that allows us to iterate through the elements of the array, one at a time. On each iteration, we will store the current array element in the variable file which we have declared, for clarity, as being of type File.
➂ Another thing we can do with Files is to check and see if they are “regular” files as opposed to being directories, links, or other special cases.
Again, this is not a special Gradle function, but is part of the normal Java behavior of a File class.
➃ The call to file.length() is just an ordinary Java function call.
➄ Several interesting things happen here.
The ‘$’ inside the quoted string allows us to request the replacement of a simple expression by its value. So we won’t actually print “file.name”. Instead the expressions file.name will be evaluated and the resulting value inserted into the string to be printed.
This “$” string substitution has been a common shortcut in scripting languages for decades.
Now, Java Files do not have a public data member called “name”, so the expression file.name would fail to compile in Java.
Java Files do, however, have a public function member getName(). And here we run into another one of those shortcuts that Groovy, as a scripting language, introduces. In Groovy, the notation x.data is considered a shorthand for x.getData() when we are trying to fetch data and for x.setData(..) when we are trying to store data. So the Groovy statement
x.data = y.member;
is actually considered shorthand for
x.setData(y.getMember());
The + operator is the conventional Java String concatenation operator.
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