在当前目录中创建 build.gradle ，内容如下：
apply plugin: 'java'
或者写成：
plugins {
id 'java'
}
执行下面的命令查看有哪些任务：
$ gradle tasks
> Task :tasks
------------------------------------------------------------
Tasks runnable from root project
------------------------------------------------------------
Build tasks
-----------
assemble - Assembles the outputs of this project.
build - Assembles and tests this project.
buildDependents - Assembles and tests this project and all projects that depend on it.
buildNeeded - Assembles and tests this project and all projects it depends on.
classes - Assembles main classes.
clean - Deletes the build directory.
jar - Assembles a jar archive containing the main classes.
testClasses - Assembles test classes.
Build Setup tasks
-----------------
init - Initializes a new Gradle build.
wrapper - Generates Gradle wrapper files.
Documentation tasks
-------------------
javadoc - Generates Javadoc API documentation for the main source code.
Help tasks
----------
buildEnvironment - Displays all buildscript dependencies declared in root project 'gradle-demo-0004'.
components - Displays the components produced by root project 'gradle-demo-0004'. [incubating]
dependencies - Displays all dependencies declared in root project 'gradle-demo-0004'.
dependencyInsight - Displays the insight into a specific dependency in root project 'gradle-demo-0004'.
dependentComponents - Displays the dependent components of components in root project 'gradle-demo-0004'. [incubating]
help - Displays a help message.
model - Displays the configuration model of root project 'gradle-demo-0004'. [incubating]
projects - Displays the sub-projects of root project 'gradle-demo-0004'.
properties - Displays the properties of root project 'gradle-demo-0004'.
tasks - Displays the tasks runnable from root project 'gradle-demo-0004'.
Verification tasks
------------------
check - Runs all checks.
test - Runs the unit tests.
Rules
-----
Pattern: clean<TaskName>: Cleans the output files of a task.
Pattern: build<ConfigurationName>: Assembles the artifacts of a configuration.
Pattern: upload<ConfigurationName>: Assembles and uploads the artifacts belonging to a configuration.
To see all tasks and more detail, run gradle tasks --all
To see more detail about a task, run gradle help --task <task>
BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed