编译器应用编程接口+注释处理==双重诊断消息？

Question

我将Java Compiler API (JSR199)与自定义批注处理器(JSR269)结合起来。提供给编译器的Java源代码中的某些语法错误会导致诊断消息出现两次。许多语法错误仍然只导致一条诊断消息。例如，不匹配的大括号只会导致一条消息，而带有无效限定标识符的导入会导致两条诊断消息，但只有在使用批注处理器时才会这样做。

以下是一些将导致问题的示例输入：

import javax.xml.bind.annotation; // missing ".*"

public class Test { }

对于批注处理器，我有以下几点。(我尝试过从process返回false，也尝试过手动实现Processor，而不是扩展AbstractProcessor。)

@SupportedSourceVersion(SourceVersion.RELEASE_7)
@SupportedAnnotationTypes("*")
public class AnnotationProcessor extends AbstractProcessor {
    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        return true;
    }
}

下面是一些用于调用编译器和添加批注处理器的代码。

javax.tools.DiagnosticCollector<javax.tools.JavaFileObject> diagnostics =
        new javax.tools.DiagnosticCollector<>();

javax.tools.StandardJavaFileManager fileManager =
        javac.getStandardFileManager(diagnostics, null, null);

fileManager.setLocation(javax.tools.StandardLocation.CLASS_OUTPUT, Arrays.asList(tempDir.toFile()));

javax.tools.JavaCompiler.CompilationTask task =
        javac.getTask(out, fileManager, diagnostics, null, null, compilationUnits);

List<Processor> processors = new ArrayList<>();
AnnotationProcessor ap = new AnnotationProcessor();
processors.add(ap);
task.setProcessors(processors);

Boolean compiled = task.call();

for (Diagnostic<? extends JavaFileObject> diag : diagnostics.getDiagnostics()) {
    out.println(diag.toString());
}

我的输出如下所示：

/Test.java:1: error: cannot find symbol
import javax.xml.bind.annotation;
                     ^
  symbol:   class annotation
  location: package javax.xml.bind
/Test.java:1: error: cannot find symbol
import javax.xml.bind.annotation;
                     ^
  symbol:   class annotation
  location: package javax.xml.bind

如果我删除processors.add(ap);行，那么重复的错误消息就会消失。添加多个处理器没有额外的效果。

你知道为什么注释处理器在使用编译器API时会导致重复的诊断消息吗？(并且只针对某些语法错误)

Answer 1

根据specification的说法，批注处理按rounds的顺序进行，并且始终至少有两轮处理。因此，示例中的Test.java被编译了两次，处理器的process方法也被调用了两次。下面是一个自包含的示例：

public class AnnotationProcessor {
    public static final String JAVA_SOURCE = "import javax.xml.bind.annotation; // missing \".*\"\n" +
            "\n" +
            "public class Test { }";

    @SupportedSourceVersion(SourceVersion.RELEASE_7)
    @SupportedAnnotationTypes("*")
    public static class MyProcessor extends AbstractProcessor {

        @Override
        public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
            System.out.println("Process : " + roundEnv);
            return true;
        }
    }

    public static void main(String[] args) throws IOException {
        JavaCompiler javac = ToolProvider.getSystemJavaCompiler();
        StandardJavaFileManager fileManager = javac.getStandardFileManager(null, null, null);
        Path source = Files.createTempDirectory("stackoverflow").resolve("Test.java");
        Files.write(source, JAVA_SOURCE.getBytes());
        Iterable<? extends JavaFileObject> compilationUnits = fileManager.getJavaFileObjects(source.toFile());
        JavaCompiler.CompilationTask task = javac.getTask(null, null, null, null, null, compilationUnits);
        task.setProcessors(Collections.singleton(new MyProcessor()));
        task.call();
    }
}

输出：

Process : [errorRaised=false, rootElements=[Test], processingOver=false]
Process : [errorRaised=false, rootElements=[], processingOver=true]
C:\Users\Alexey_2\AppData\Local\Temp\stackoverflow4274340620494105881\Test.java:1: error: cannot find symbol
import javax.xml.bind.annotation; // missing ".*"
                     ^
  symbol:   class annotation
  location: package javax.xml.bind
C:\Users\Alexey_2\AppData\Local\Temp\stackoverflow4274340620494105881\Test.java:1: error: cannot find symbol
import javax.xml.bind.annotation; // missing ".*"
                     ^
  symbol:   class annotation
  location: package javax.xml.bind
1 error