The compiler might generate diagnostics during compilation (for example, error messages). If a diagnostic listener is provided, the diagnostics will be supplied to the listener. If no listener is provided, the diagnostics will be formatted in an unspecified format and written to the default output, which is System.err unless otherwise specified. Even if a diagnostic listener is supplied, some diagnostics might not fit in a Diagnostic and will be written to the default output.

A compiler tool has an associated standard file manager, which is the file manager that is native to the tool (or built-in). The standard file manager can be obtained by calling getStandardFileManager.

A compiler tool must function with any file manager as long as any additional requirements as detailed in the methods below are met. If no file manager is provided, the compiler tool will use a standard file manager such as the one returned by getStandardFileManager.

An instance implementing this interface must conform to The Java Language Specification and generate class files conforming to The Java Virtual Machine Specification. The versions of these specifications are defined in the Tool interface. Additionally, an instance of this interface supporting SourceVersion.RELEASE_6 or higher must also support annotation processing.

The compiler relies on two services: diagnostic listener and file manager. Although most classes and interfaces in this package defines an API for compilers (and tools in general) the interfaces DiagnosticListener, JavaFileManager, FileObject, and JavaFileObject are not intended to be used in applications. Instead these interfaces are intended to be implemented and used to provide customized services for a compiler and thus defines an SPI for compilers.

There are a number of classes and interfaces in this package which are designed to ease the implementation of the SPI to customize the behavior of a compiler:

StandardJavaFileManager

Every compiler which implements this interface provides a standard file manager for operating on regular files. The StandardJavaFileManager interface defines additional methods for creating file objects from regular files.

The standard file manager serves two purposes:

• basic building block for customizing how a compiler reads and writes files
• sharing between multiple compilation tasks

Reusing a file manager can potentially reduce overhead of scanning the file system and reading jar files. Although there might be no reduction in overhead, a standard file manager must work with multiple sequential compilations making the following example a recommended coding pattern:

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File[] files1 = ... ; // input for first compilation task
File[] files2 = ... ; // input for second compilation task

JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager fileManager = compiler.getStandardFileManager(null, null, null);

Iterable<? extends JavaFileObject> compilationUnits1 =
    fileManager.getJavaFileObjectsFromFiles(Arrays.asList(files1));
compiler.getTask(null, fileManager, null, null, null, compilationUnits1).call();

Iterable<? extends JavaFileObject> compilationUnits2 =
    fileManager.getJavaFileObjects(files2); // use alternative method
// reuse the same file manager to allow caching of jar files
compiler.getTask(null, fileManager, null, null, null, compilationUnits2).call();

fileManager.close();

DiagnosticCollector

Used to collect diagnostics in a list, for example:

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Iterable<? extends JavaFileObject> compilationUnits = ...;
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<JavaFileObject>();
StandardJavaFileManager fileManager = compiler.getStandardFileManager(diagnostics, null, null);
compiler.getTask(null, fileManager, diagnostics, null, null, compilationUnits).call();

for (Diagnostic<? extends JavaFileObject> diagnostic : diagnostics.getDiagnostics()) {
    System.out.format("Error on line %d in %s%n",
                      diagnostic.getLineNumber(),
                      diagnostic.getSource().toUri());
}

fileManager.close();

ForwardingJavaFileManager, ForwardingFileObject, and ForwardingJavaFileObject

Subclassing is not available for overriding the behavior of a standard file manager as it is created by calling a method on a compiler, not by invoking a constructor. Instead forwarding (or delegation) should be used. These classes makes it easy to forward most calls to a given file manager or file object while allowing customizing behavior. For example, consider how to log all calls to JavaFileManager.flush():

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final  Logger logger = ...;
Iterable<? extends JavaFileObject> compilationUnits = ...;
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager stdFileManager = compiler.getStandardFileManager(null, null, null);
JavaFileManager fileManager = new ForwardingJavaFileManager(stdFileManager) {
    @Override
    public void flush() throws IOException {
        logger.entering(StandardJavaFileManager.class.getName(), "flush");
        super.flush();
        logger.exiting(StandardJavaFileManager.class.getName(), "flush");
    }
};
compiler.getTask(null, fileManager, null, null, null, compilationUnits).call();

SimpleJavaFileObject

This class provides a basic file object implementation which can be used as building block for creating file objects. For example, here is how to define a file object which represent source code stored in a string:

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/**
 * A file object used to represent source coming from a string.
 */
public class JavaSourceFromString extends SimpleJavaFileObject {
    /**
     * The source code of this "file".
     */
    final String code;

    /**
     * Constructs a new JavaSourceFromString.
     * @param name the name of the compilation unit represented by this file object
     * @param code the source code for the compilation unit represented by this file object
     */
    JavaSourceFromString(String name, String code) {
        super(URI.create("string:///" + name.replace('.','/') + Kind.SOURCE.extension),
              Kind.SOURCE);
        this.code = code;
    }

    @Override
    public CharSequence getCharContent(boolean ignoreEncodingErrors) {
        return code;
    }
}