Javac的语法糖

Posted 2020-10-03

 

在Javac中解语法糖主要是Lower类来完成，调用这个类的入口函数translateTopLevelClass即可。这个方法只是JavacCompiler类的desugar方法中进行了调用，desugar方法的具体代码如下：

/**
     * Prepare attributed parse trees, in conjunction with their attribution contexts,
     * for source or code generation. If the file was not listed on the command line,
     * the current implicitSourcePolicy is taken into account.
     * The preparation stops as soon as an error is found.
     */
    protected void desugar(final Env<AttrContext> env, Queue<Pair<Env<AttrContext>, JCClassDecl>> results) {
        if (shouldStop(CompileState.TRANSTYPES))
            return;

        if (implicitSourcePolicy == ImplicitSourcePolicy.NONE
                && !inputFiles.contains(env.toplevel.sourcefile)) {
            return;
        }

        if (compileStates.isDone(env, CompileState.LOWER)) {
            results.addAll(desugaredEnvs.get(env));
            return;
        }

        /**
         * Ensure that superclasses of C are desugared before C itself. This is
         * required for two reasons: (i) as erasure (TransTypes) destroys
         * information needed in flow analysis and (ii) as some checks carried
         * out during lowering require that all synthetic fields/methods have
         * already been added to C and its superclasses.
         */
        class ScanNested extends TreeScanner {
            Set<Env<AttrContext>> dependencies = new LinkedHashSet<Env<AttrContext>>();
            @Override
            public void visitClassDef(JCClassDecl node) {
                Type st = types.supertype(node.sym.type);
                if (st.tag == TypeTags.CLASS) {
                    ClassSymbol c = st.tsym.outermostClass();
                    Env<AttrContext> stEnv = enter.getEnv(c);
                    if (stEnv != null && env != stEnv) {
                        if (dependencies.add(stEnv))
                            scan(stEnv.tree);
                    }
                }
                super.visitClassDef(node);
            }
        }
        ScanNested scanner = new ScanNested();
        scanner.scan(env.tree);
        for (Env<AttrContext> dep: scanner.dependencies) {
        if (!compileStates.isDone(dep, CompileState.FLOW))
            desugaredEnvs.put(dep, desugar(flow(attribute(dep))));
        }

        //We need to check for error another time as more classes might
        //have been attributed and analyzed at this stage
        if (shouldStop(CompileState.TRANSTYPES))
            return;

        if (verboseCompilePolicy)
            printNote("[desugar " + env.enclClass.sym + "]");

        JavaFileObject prev = log.useSource(env.enclClass.sym.sourcefile != null ?
                                  env.enclClass.sym.sourcefile :
                                  env.toplevel.sourcefile);
        try {
            //save tree prior to rewriting
            JCTree untranslated = env.tree;

            make.at(Position.FIRSTPOS);
            TreeMaker localMake = make.forToplevel(env.toplevel);

            if (env.tree instanceof JCCompilationUnit) {
                if (!(stubOutput || sourceOutput || printFlat)) {
                    if (shouldStop(CompileState.LOWER))
                        return;
                    List<JCTree> pdef = lower.translateTopLevelClass(env, env.tree, localMake);
                    if (pdef.head != null) {
                        Assert.check(pdef.tail.isEmpty());
                        results.add(new Pair<Env<AttrContext>, JCClassDecl>(env, (JCClassDecl)pdef.head));
                    }
                }
                return;
            }

            if (stubOutput) {
                //emit stub Java source file, only for compilation
                //units enumerated explicitly on the command line
                JCClassDecl cdef = (JCClassDecl)env.tree;
                if (untranslated instanceof JCClassDecl &&
                    rootClasses.contains((JCClassDecl)untranslated) &&
                    ((cdef.mods.flags & (Flags.PROTECTED|Flags.PUBLIC)) != 0 ||
                     cdef.sym.packge().getQualifiedName() == names.java_lang)) {
                    results.add(new Pair<Env<AttrContext>, JCClassDecl>(env, removeMethodBodies(cdef)));
                }
                return;
            }

            if (shouldStop(CompileState.TRANSTYPES))
                return;

            env.tree = transTypes.translateTopLevelClass(env.tree, localMake);
            compileStates.put(env, CompileState.TRANSTYPES);

            if (shouldStop(CompileState.LOWER))
                return;

            if (sourceOutput) {
                //emit standard Java source file, only for compilation
                //units enumerated explicitly on the command line
                JCClassDecl cdef = (JCClassDecl)env.tree;
                if (untranslated instanceof JCClassDecl &&
                    rootClasses.contains((JCClassDecl)untranslated)) {
                    results.add(new Pair<Env<AttrContext>, JCClassDecl>(env, cdef));
                }
                return;
            }

            //translate out inner classes
            List<JCTree> cdefs = lower.translateTopLevelClass(env, env.tree, localMake);
            compileStates.put(env, CompileState.LOWER);

            if (shouldStop(CompileState.LOWER))
                return;

            //generate code for each class
            for (List<JCTree> l = cdefs; l.nonEmpty(); l = l.tail) {
                JCClassDecl cdef = (JCClassDecl)l.head;
                results.add(new Pair<Env<AttrContext>, JCClassDecl>(env, cdef));
            }
        }
        finally {
            log.useSource(prev);
        }

    }