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android-notes/blogs/Android/Gradle/依赖实现分析.md

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---
深入理解 Gradle 框架之 依赖实现分析
---
> 本文摘自:@ AllenWang [深入理解gradle框架之二:依赖实现分析](https://mp.weixin.qq.com/s/WQCqUaYDPHDIjUHlxjRIkw)
### 目录
1. 引言
2. 从 implementation 说起
### 引言
大家在日常开发中,见过最多的可能就是下面三种依赖声明:
```groovy
implementation org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version
implementation project(":applemodule")
implementation fileTree(dir:libs, include:[*.jar])
```
这些不同的依赖声明,内部实现机制是怎么样的?
### 从 implementation 说起
```
implementation 'com.android.support:appcompat-v7:28.0.0'
```
按照 Groovy 的语法,这里要执行的是 DependencyHandler 的 implementation() 方法,参数为 'com.android.support:appcompat-v7:28.0.0'。
但是在 DependencyHandler 中并没有 implementation() 这个方法,这是怎么回事呢?
```java
public interface DependencyHandler {
Dependency add(String configurationName, Object dependencyNotation);
Dependency create(Object dependencyNotation);
Dependency module(Object notation);
Dependency project(Map<String, ?> notation);
Dependency gradleApi();
Dependency localGroovy();
//...
}
```
#### MethodMissing
这其实涉及到了 Groovy 语言的一个重要特性:methodMissing,这个特性允许在运行时 catch 对于未定义方法的调用。
Gradle 对这个特性进行封装,一个类要想使用这个特性,只要实现 MixIn 接口即可,这个接口如下:
```java
public interface MethodMixIn {
MethodAccess getAdditionalMethods();
}
```
其中 MethodAccess 接口如下:
```java
public interface MethodAccess {
boolean hasMethod(String var1, Object... var2);
DynamicInvokeResult tryInvokeMethod(String var1, Object... var2);
}
```
也就是说,对于 DependencyHandler 中未定义的方法(如 implementation() 方法),只要 hasMethod 返回 true,就会调用到 MethodAccess 的实现者的 tryInvokeMethod 方法,其中 name 为 configuration 名称,arguments 就是 'com.android.support:appcompat-v7:28.0.0' 这个参数。
那 DependencyHandler 接口的实现者 DefaultDependencyHandler 是如何实现 MethodMixIn 这个接口的呢?
```java
public MethodAccess getAdditionalMethods() {
return this.dynamicMethods;
}
```
这个 dynamicMethods 类型为 DynamicAddDependencyMethods,源码如下:
```java
class DynamicAddDependencyMethods implements MethodAccess {
private ConfigurationContainer configurationContainer;
private DynamicAddDependencyMethods.DependencyAdder dependencyAdder;
DynamicAddDependencyMethods(ConfigurationContainer configurationContainer, DynamicAddDependencyMethods.DependencyAdder dependencyAdder) {
this.configurationContainer = configurationContainer;
this.dependencyAdder = dependencyAdder;
}
public boolean hasMethod(String name, Object... arguments) {
return arguments.length != 0 && this.configurationContainer.findByName(name) != null;
}
public DynamicInvokeResult tryInvokeMethod(String name, Object... arguments) {
if (arguments.length == 0) {
return DynamicInvokeResult.notFound();
} else {
Configuration configuration = (Configuration)this.configurationContainer.findByName(name);
if (configuration == null) {
return DynamicInvokeResult.notFound();
} else {
List<?> normalizedArgs = CollectionUtils.flattenCollections(arguments);
if (normalizedArgs.size() == 2 && normalizedArgs.get(1) instanceof Closure) {
return DynamicInvokeResult.found(this.dependencyAdder.add(configuration, normalizedArgs.get(0), (Closure)normalizedArgs.get(1)));
} else if (normalizedArgs.size() == 1) {
return DynamicInvokeResult.found(this.dependencyAdder.add(configuration, normalizedArgs.get(0), (Closure)null));
} else {
Iterator var5 = normalizedArgs.iterator();
while(var5.hasNext()) {
Object arg = var5.next();
this.dependencyAdder.add(configuration, arg, (Closure)null);
}
return DynamicInvokeResult.found();
}
}
}
}
interface DependencyAdder<T> {
T add(Configuration var1, Object var2, Closure var3);
}
}
```
注意到它是实现了 MethodAccess 这个接口的,首先看它的 hasMethod() 方法,很简单,返回 true 的条件是:
* 参数长度不为零
* configuration 必须是已经定义过的
然后再看 tryInvokeMethod(),它会先通过 configurationsContainer 找到对应的 configuration,然后分如下几种情况:
* 参数个数为 2,并且第 2 个参数是 Closure
* 参数个数为 1
* 其他情形
不过不管哪种情形,都会先调用 dependencyAdder 的 add() 方法,而 dependencyAdder 是 DefaultDependencyHandler.DirectDependencyAdder 对象,其 add() 方法如下:
```java
public Dependency add(Configuration configuration, Object dependencyNotation, @Nullable Closure configureAction) {
return DefaultDependencyHandler.this.doAdd(configuration, dependencyNotation, configureAction);
}
```
可见,其实是调用外部类 DefaultDependencyHandler 的 doAdd() 方法,在下一小节分析该方法。
#### DefaultDependencyHandler.doAdd() 方法分析
```java
private Dependency doAdd(Configuration configuration, Object dependencyNotation, Closure configureClosure) {
if (dependencyNotation instanceof Configuration) {
Configuration other = (Configuration)dependencyNotation;
if (!this.configurationContainer.contains(other)) {
throw new UnsupportedOperationException("Currently you can only declare dependencies on configurations from the same project.");
} else {
configuration.extendsFrom(new Configuration[]{other});
return null;
}
} else {
Dependency dependency = this.create(dependencyNotation, configureClosure);
configuration.getDependencies().add(dependency);
return dependency;
}
}
```
可见,这里会先判断 dependencyNotation 是否为 Configuration,如果是的话,就让当前的 configuration 继承自 other 这个 configuration,而继承的意思就是,后续所有添加到 other 的依赖,也会添加到当前这个 configuration 中。
为什么还要考虑参数中的 dependecnyNotation 是否为 Configuration 的情形呢?
其实就是考虑到有诸如 implementation project(path: ':appmodule', configuration: 'configA') 这样的依赖声明。
至于依赖的创建过程,在下一节进行分析。
### 依赖创建过程分析
DefaultDependencyHandler 的 create() 方法如下:
```java
public Dependency create(Object dependencyNotation, Closure configureClosure) {
Dependency dependency = this.dependencyFactory.createDependency(dependencyNotation);
return (Dependency)ConfigureUtil.configure(configureClosure, dependency);
}
```
其中的 dependencyFactory 为 DefaultDependencyFactory 对象,其 createDependency() 方法如下:
```java
public Dependency createDependency(Object dependencyNotation) {
Dependency dependency = (Dependency)this.dependencyNotationParser.parseNotation(dependencyNotation);
this.injectAttributesFactory(dependency);
return dependency;
}
```
可见,它是直接调用 dependencyNotationParser 这个解析器对于 denpendencyNotation 进行解析。
其中的 dependencyNotationParser 是实现了接口 NotationParser\<Object, Dependency> 接口的对象。
为了找出这里的 dependencyNotationParser 到底是哪个类的实例,查看 DefaultDependencyFactory 的创建,在 DependencyManagementBuildScopeServices 类中,如下:
```java
DependencyFactory createDependencyFactory(Instantiator instantiator, ProjectAccessListener projectAccessListener, StartParameter startParameter, ClassPathRegistry classPathRegistry, CurrentGradleInstallation currentGradleInstallation, FileLookup fileLookup, RuntimeShadedJarFactory runtimeShadedJarFactory, ImmutableAttributesFactory attributesFactory, SimpleMapInterner stringInterner) {
DefaultProjectDependencyFactory factory = new DefaultProjectDependencyFactory(projectAccessListener, instantiator, startParameter.isBuildProjectDependencies());
ProjectDependencyFactory projectDependencyFactory = new ProjectDependencyFactory(factory);
return new DefaultDependencyFactory(DependencyNotationParser.parser(instantiator, factory, classPathRegistry, fileLookup, runtimeShadedJarFactory, currentGradleInstallation, stringInterner), DependencyConstraintNotationParser.parser(instantiator, stringInterner), (new ClientModuleNotationParserFactory(instantiator, stringInterner)).create(), projectDependencyFactory, attributesFactory);
}
```
可见,它是通过 DependencyNotationParser.parser() 方法创建的,该方法如下:
```java
public static NotationParser<Object, Dependency> parser(Instantiator instantiator, DefaultProjectDependencyFactory dependencyFactory, ClassPathRegistry classPathRegistry, FileLookup fileLookup, RuntimeShadedJarFactory runtimeShadedJarFactory, CurrentGradleInstallation currentGradleInstallation, Interner<String> stringInterner) {
return NotationParserBuilder.toType(Dependency.class).fromCharSequence(new DependencyStringNotationConverter(instantiator, DefaultExternalModuleDependency.class, stringInterner)).converter(new DependencyMapNotationConverter(instantiator, DefaultExternalModuleDependency.class)).fromType(FileCollection.class, new DependencyFilesNotationConverter(instantiator)).fromType(Project.class, new DependencyProjectNotationConverter(dependencyFactory)).fromType(ClassPathNotation.class, new DependencyClassPathNotationConverter(instantiator, classPathRegistry, fileLookup.getFileResolver(), runtimeShadedJarFactory, currentGradleInstallation)).invalidNotationMessage("Comprehensive documentation on dependency notations is available in DSL reference for DependencyHandler type.").toComposite();
}
```
这个方法其实很好理解,它其实是创建了多个实现了接口 NotationConverter 的对象,然后将这些转换器都添加在一起,构成一个综合的转换器。
其中,DependencyStringNotationConvert 负责将字符串类型的 notation 转换为 DefaultExternalModuleDependency,也就是对应 implementation 'com.android.support:appcompat-v7:25.1.0' 这样的声明;
DependencyFilesNotationConverter 将 FileCollection 转换为 SelfResolvingDependency,也就是对应的 implementation fileTree(dir: 'libs', include:['*.jar']) 这样的声明;
DependencyProjectNotationConverter 将 Project 转换为 ProjectDependency,对应 implementation project(":appmodule") 这样的情形;
DependencyClasspathNotationConvert 将 ClasspathNotation 转换为 SelfResolvingDependency;
到这里,就知道类似 implementation 'com.android.support:appcompat-v7:28.0.0' 和 implementation project 这样的声明,其实是被不同的转换器,转换成了 SelfResolvingDependency 或者 ProjectDependency。
这里可以看出,除了 project 依赖之外,其他都转换成 SelfResolvingDependency,所谓的 SelfResolvingDependency 其实是可以自解析的依赖,独立于 repository。
ProjectDependency 则不然,它是依赖于 repository 的,下面就分析 ProjectDependency 的独特之处。