gradle这东西new groovy.json

Gradle是一个非常好用的编译工具，特别是继承了maven的依赖项管理功能，需要的Library不需要像传统IDE一样手动下载复制到项目中，只需要简单的写一行gradle脚本，就能自动下载下来并编译。
但是有时候会出现各种不明情况的报错，最常见的一种原因就是依赖项版本冲突。
每个模块都可能依赖其他模块，这些模块又会依赖别的模块。而一个项目中的多个模块，对同一个模块的不同版本有依赖，就可能产生冲突。

通过gradle命令查看依赖树，可以比较直观的看到冲突。具体方法是在模块所在的目录，也即build.gradle所在目录下执行gradle dependencies（需要将gradle加入PATH环境变量）

Transitive
Transitive用于自动处理子依赖项。默认为true，gradle自动添加子依赖项，形成一个多层树形结构；设置为false，则需要手动添加每个依赖项。
案例
以安卓单元测试espresso的配置为例，gradle依赖如下：
dependencies {
androidTestCompile('com.android.support.test:runner:0.2')
androidTestCompile('com.android.support.test:rules:0.2')
androidTestCompile('com.android.support.test.espresso:espresso-core:2.1')
}
运行gradle dependencies的结果如下。可以看到每个包的依赖项都被递归分析并添加进来。
+        --- com.android.support.test:runner:0.2
| +--- junit:junit-dep:4.10
| | \--- org.hamcrest:hamcrest-core:1.1
| +--- com.android.support.test:exposed-instrumentation-api-publish:0.2
| \--- com.android.support:support-annotations:22.0.0
+        --- com.android.support.test:rules:0.2
| \--- com.android.support.test:runner:0.2 (*)
\         --- com.android.support.test.espresso:espresso-core:2.1
+--- com.android.support.test:rules:0.2 (*)
+--- com.squareup:javawriter:2.1.1
+--- org.hamcrest:hamcrest-integration:1.1
| \--- org.hamcrest:hamcrest-core:1.1
+--- com.android.support.test.espresso:espresso-idling-resource:2.1
+--- org.hamcrest:hamcrest-library:1.1
| \--- org.hamcrest:hamcrest-core:1.1
+--- javax.inject:javax.inject:1
+--- com.google.code.findbugs:jsr305:2.0.1
+--- com.android.support.test:runner:0.2 (*)
+--- javax.annotation:javax.annotation-api:1.2
\--- org.hamcrest:hamcrest-core:1.1
统一指定transitive

可以给dependencies统一指定transitive为false，再次执行dependencies可以看到如下结果。

configurations.all {
transitive = false
}
dependencies {
androidTestCompile('com.android.support.test:runner:0.2')
androidTestCompile('com.android.support.test:rules:0.2')
androidTestCompile('com.android.support.test.espresso:espresso-core:2.1')
}
+--- com.android.support.test:runner:0.2
+--- com.android.support.test:rules:0.2
\--- com.android.support.test.espresso:espresso-core:2.1
单独指定依赖项的transitive

dependencies {
androidTestCompile('com.android.support.test:runner:0.2')
androidTestCompile('com.android.support.test:rules:0.2')
androidTestCompile('com.android.support.test.espresso:espresso-core:2.1') {
transitive = false
}
}
版本冲突

在同一个配置下（例如androidTestCompile），某个模块的不同版本同时被依赖时，默认使用最新版，gradle同步时不会报错。例如下面的hamcrest-core和runner。

dependencies {
androidTestCompile('com.android.support.test:runner:0.4')
androidTestCompile('com.android.support.test:rules:0.2')
androidTestCompile('com.android.support.test.espresso:espresso-core:2.1')
}
+--- com.android.support.test:runner:0.4
| +--- com.android.support:support-annotations:23.0.1
| +--- junit:junit:4.12
| | \--- org.hamcrest:hamcrest-core:1.3
| \--- com.android.support.test:exposed-instrumentation-api-publish:0.4
+--- com.android.support.test:rules:0.2
| \--- com.android.support.test:runner:0.2 -> 0.4 (*)
\--- com.android.support.test.espresso:espresso-core:2.1
+--- com.android.support.test:rules:0.2 (*)
+--- com.squareup:javawriter:2.1.1
+--- org.hamcrest:hamcrest-integration:1.1
| \--- org.hamcrest:hamcrest-core:1.1 -> 1.3
+--- com.android.support.test.espresso:espresso-idling-resource:2.1
+--- org.hamcrest:hamcrest-library:1.1
| \--- org.hamcrest:hamcrest-core:1.1 -> 1.3
+--- javax.inject:javax.inject:1
+--- com.google.code.findbugs:jsr305:2.0.1
+--- com.android.support.test:runner:0.2 -> 0.4 (*)
+--- javax.annotation:javax.annotation-api:1.2
\--- org.hamcrest:hamcrest-core:1.1 -> 1.3
Force

force强制设置某个模块的版本。

configurations.all {
resolutionStrategy {
force 'org.hamcrest:hamcrest-core:1.3'
}
}
dependencies {
androidTestCompile('com.android.support.test:runner:0.2')
androidTestCompile('com.android.support.test:rules:0.2')
androidTestCompile('com.android.support.test.espresso:espresso-core:2.1')
}
可以看到，原本对hamcrest-core 1.1的依赖，全部变成了1.3。

+--- com.android.support.test:runner:0.2
| +--- junit:junit-dep:4.10
| | \--- org.hamcrest:hamcrest-core:1.1 -> 1.3
| +--- com.android.support.test:exposed-instrumentation-api-publish:0.2
| \--- com.android.support:support-annotations:22.0.0
+--- com.android.support.test:rules:0.2
| \--- com.android.support.test:runner:0.2 (*)
\--- com.android.support.test.espresso:espresso-core:2.1
+--- com.android.support.test:rules:0.2 (*)
+--- com.squareup:javawriter:2.1.1
+--- org.hamcrest:hamcrest-integration:1.1
| \--- org.hamcrest:hamcrest-core:1.1 -> 1.3
+--- com.android.support.test.espresso:espresso-idling-resource:2.1
+--- org.hamcrest:hamcrest-library:1.1
| \--- org.hamcrest:hamcrest-core:1.1 -> 1.3
+--- javax.inject:javax.inject:1
+--- com.google.code.findbugs:jsr305:2.0.1
+--- com.android.support.test:runner:0.2 (*)
+--- javax.annotation:javax.annotation-api:1.2
\--- org.hamcrest:hamcrest-core:1.1 -> 1.3
Exclude

Exclude可以设置不编译指定的模块

configurations {
all*.exclude group: 'org.hamcrest', module: 'hamcrest-core'
}
dependencies {
androidTestCompile('com.android.support.test:runner:0.2')
androidTestCompile('com.android.support.test:rules:0.2')
androidTestCompile('com.android.support.test.espresso:espresso-core:2.1')
}
+--- com.android.support.test:runner:0.2
| +--- junit:junit-dep:4.10
| +--- com.android.support.test:exposed-instrumentation-api-publish:0.2
| \--- com.android.support:support-annotations:22.0.0
+--- com.android.support.test:rules:0.2
| \--- com.android.support.test:runner:0.2 (*)
\--- com.android.support.test.espresso:espresso-core:2.1
+--- com.android.support.test:rules:0.2 (*)
+--- com.squareup:javawriter:2.1.1
+--- org.hamcrest:hamcrest-integration:1.1
+--- com.android.support.test.espresso:espresso-idling-resource:2.1
+--- org.hamcrest:hamcrest-library:1.1
+--- javax.inject:javax.inject:1
+--- com.google.code.findbugs:jsr305:2.0.1
+--- com.android.support.test:runner:0.2 (*)
\--- javax.annotation:javax.annotation-api:1.2
单独使用group或module参数

exclude后的参数有group和module，可以分别单独使用，会排除所有匹配项。例如下面的脚本匹配了所有的group为’com.android.support.test’的模块。

configurations {
all*.exclude group: 'com.android.support.test'
}
dependencies {
androidTestCompile('com.android.support.test:runner:0.2')
androidTestCompile('com.android.support.test:rules:0.2')
androidTestCompile('com.android.support.test.espresso:espresso-core:2.1')
}
\--- com.android.support.test.espresso:espresso-core:2.1
+--- com.squareup:javawriter:2.1.1
+--- org.hamcrest:hamcrest-integration:1.1
| \--- org.hamcrest:hamcrest-core:1.1
+--- com.android.support.test.espresso:espresso-idling-resource:2.1
+--- org.hamcrest:hamcrest-library:1.1
| \--- org.hamcrest:hamcrest-core:1.1
+--- javax.inject:javax.inject:1
+--- com.google.code.findbugs:jsr305:2.0.1
+--- javax.annotation:javax.annotation-api:1.2
\--- org.hamcrest:hamcrest-core:1.1
单独给某个模块指定exclude

dependencies {
androidTestCompile('com.android.support.test:runner:0.2')
androidTestCompile('com.android.support.test:rules:0.2')
androidTestCompile('com.android.support.test.espresso:espresso-core:2.1') {
exclude group: 'org.hamcrest'
}
}
+--- com.android.support.test:runner:0.2
| +--- junit:junit-dep:4.10
| | \--- org.hamcrest:hamcrest-core:1.1
| +--- com.android.support.test:exposed-instrumentation-api-publish:0.2
| \--- com.android.support:support-annotations:22.0.0
+--- com.android.support.test:rules:0.2
| \--- com.android.support.test:runner:0.2 (*)
\--- com.android.support.test.espresso:espresso-core:2.1
+--- com.android.support.test:rules:0.2 (*)
+--- com.squareup:javawriter:2.1.1
+--- com.android.support.test.espresso:espresso-idling-resource:2.1
+--- javax.inject:javax.inject:1
+--- com.google.code.findbugs:jsr305:2.0.1
+--- com.android.support.test:runner:0.2 (*)
\--- javax.annotation:javax.annotation-api:1.2
不同配置下的版本冲突
同样的配置下的版本冲突，会自动使用最新版；而不同配置下的版本冲突，gradle同步时会直接报错。可使用exclude、force解决冲突。

不兼容
虽然可以通过force、exclude等方式避免依赖项版本冲突，使得grade同步成功，但是并不能代表编译时没有问题。由于不同版本可能不完全兼容，于是会出现各种奇怪的报错。已知的解决思路是更改包的版本、尝试强制使用不同版本的依赖项，找到可兼容的依赖组合。