看下 Spring boot tools 子项目包含的内容：
重点工具介绍
1、spring-boot-annotation-processor
2、spring-boot-maven-plugin
3、spring-boot-loader
1、Spring Boot Annotation Processor
Annotation Processor 是一种利用java 注解 扩展javac 编译功能的一种方式。
定义一个Processor ，Processor 可以通过 javac 指定参数 类名的方式获取，也可以使用服务发现的方式，javac 会自动扫描类路径下面 META-INF/services/javax.annotation.processing.Processor文件里面的实现类。spring boot 使用后者的方式，所以每次项目编译，都会触发Processor 里面的逻辑。我们可以使用 JavaCompiler 类对书写的Processor 进行功能测试。详见 Spring boot
org.springframework.boot.testsupport.compiler.TestCompiler
javac 命令说明：
javac -help
-processor <class1>[,<class2>,<class3>...] 要运行的注释处理程序的名称; 绕过默认的搜索进程
-processorpath <路径> 指定查找注释处理程序的位置
测试代码：
1、定义一个BuilderProcessor 自动生成POJO 的builder
public class Person {
private int age;
private String name;
// getters and setters …
}
通过BuilderProcessor 在编译时候生成如下类。
Person person = new PersonBuilder()
.setAge(25)
.setName("John")
.build();
0、定义一个注解
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.SOURCE)
public @interface BuilderProperty {
}
1、BuilderProcessor 实现 抽象类AbstractProcessor
@SupportedAnnotationTypes(
"com.github.yulechen.BuilderProperty")
@SupportedSourceVersion(SourceVersion.RELEASE_8)
public class BuilderProcessor extends AbstractProcessor {
@Override
public boolean process(Set<? extends TypeElement> annotations,
RoundEnvironment roundEnv) {
System.out.println("start process java source files");
for (TypeElement annotation : annotations) {
Set<? extends Element> annotatedElements
= roundEnv.getElementsAnnotatedWith(annotation);
Map<Boolean, List<Element>> annotatedMethods = annotatedElements.stream().collect(
Collectors.partitioningBy(element ->
((ExecutableType) element.asType()).getParameterTypes().size() == 1
&& element.getSimpleName().toString().startsWith("set")));
List<Element> setters = annotatedMethods.get(true);
List<Element> otherMethods = annotatedMethods.get(false);
otherMethods.forEach(element ->
processingEnv.getMessager().printMessage(Diagnostic.Kind.ERROR,
"@BuilderProperty must be applied to a setXxx method "
+ "with a single argument", element));
if (setters.isEmpty()) {
continue;
}
String className = ((TypeElement) setters.get(0)
.getEnclosingElement()).getQualifiedName().toString();
Map<String, String> setterMap = setters.stream().collect(Collectors.toMap(
setter -> setter.getSimpleName().toString(),
setter -> ((ExecutableType) setter.asType())
.getParameterTypes().get(0).toString()
));
try {
writeBuilderFile(className,setterMap);
} catch (IOException e) {
e.printStackTrace();
}
}
return true;
}
private void writeBuilderFile(
String className, Map<String, String> setterMap)
throws IOException {
String packageName = null;
int lastDot = className.lastIndexOf('.');
if (lastDot > 0) {
packageName = className.substring(0, lastDot);
}
String simpleClassName = className.substring(lastDot + 1);
String builderClassName = className + "Builder";
String builderSimpleClassName = builderClassName
.substring(lastDot + 1);
JavaFileObject builderFile = processingEnv.getFiler()
.createSourceFile(builderClassName);
try (PrintWriter out = new PrintWriter(builderFile.openWriter())) {
if (packageName != null) {
out.print("package ");
out.print(packageName);
out.println(";");
out.println();
}
out.print("public class ");
out.print(builderSimpleClassName);
out.println(" {");
out.println();
out.print(" private ");
out.print(simpleClassName);
out.print(" object = new ");
out.print(simpleClassName);
out.println("();");
out.println();
out.print(" public ");
out.print(simpleClassName);
out.println(" build() {");
out.println(" return object;");
out.println(" }");
out.println();
setterMap.entrySet().forEach(setter -> {
String methodName = setter.getKey();
String argumentType = setter.getValue();
out.print(" public ");
out.print(builderSimpleClassName);
out.print(" ");
out.print(methodName);
out.print("(");
out.print(argumentType);
out.println(" value) {");
out.print(" object.");
out.print(methodName);
out.println("(value);");
out.println(" return this;");
out.println(" }");
out.println();
});
out.println("}");
}
}
}
2、建一个服务发现文件 META-INF/services/javax.annotation.processing.Processor ,文件里面为BuilderProcessor 全路径名称。
3、将0，1，2 编译成jar 包，供其他项目引用。加入jar 包名称为processor.jar
4、另一个项目引用processor.jar
public class Person {
private int age;
private String name;
public int getAge() {
return age;
}
@BuilderProperty
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
@BuilderProperty
public void setName(String name) {
this.name = name;
}
}
编译Person 文件
javac $XXXPATH/Person.java -cp $XXXLIBPATH/processor.jar
编译结果，产生了一个PersonBuilder
Spring boot 有两个processor
1、spring-boot-configuration-annotation-processor 项目下面的 ConfigurationMetadataAnnotationProcessor 会扫描属性相关的注解
ConfigurationProperties，NestedConfigurationProperty，DeprecatedConfigurationProperty。然后生成 META-INF/spring-configuration-metadata.json 文件。
protected ConfigurationMetadata writeMetaData() throws Exception {
ConfigurationMetadata metadata = this.metadataCollector.getMetadata();
metadata = mergeAdditionalMetadata(metadata);
if (!metadata.getItems().isEmpty()) {
this.metadataStore.writeMetadata(metadata);
return metadata;
}
return null;
}
2、spring-boot-autoconfigure-annotation-processor 项目下面AutoConfigureAnnotationProcessor ，它处理的注解有
@ConditionalOnClass
@ConditionalOnBean
@ConditionalOnSingleCandidate
@ConditionalOnWebApplication
@AutoConfigureBefore
@AutoConfigureAfter
@AutoConfigureOrder
编译处理逻辑：
生成一个"META-INF/spring-autoconfigure-metadata.properties" 文件。
private void writeProperties() throws IOException {
if (!this.properties.isEmpty()) {
Filer filer = this.processingEnv.getFiler();
FileObject file = filer.createResource(StandardLocation.CLASS_OUTPUT, "", PROPERTIES_PATH);
try (Writer writer = new OutputStreamWriter(file.openOutputStream(), StandardCharsets.UTF_8)) {
for (Map.Entry<String, String> entry : this.properties.entrySet()) {
writer.append(entry.getKey());
writer.append("=");
writer.append(entry.getValue());
writer.append(System.lineSeparator());
}
}
}
}
