这一篇重点来总结下javac的访问者模式，其定义的访问者接口为JCTree.Visitor，具体如下：
/** A generic visitor class for trees.
*/
public static abstract class Visitor {
public void visitTopLevel(JCCompilationUnit that) { visitTree(that); }
public void visitImport(JCImport that) { visitTree(that); }
public void visitClassDefinition(JCClassDeclaration that) { visitTree(that); }
public void visitMethodDefinition(JCMethodDeclaration that) { visitTree(that); }
public void visitVariableDefinition(JCVariableDeclaration that) { visitTree(that); }
public void visitSkip(JCSkip that) { visitTree(that); }
public void visitBlock(JCBlock that) { visitTree(that); }
public void visitDoLoop(JCDoWhileLoop that) { visitTree(that); }
public void visitWhileLoop(JCWhileLoop that) { visitTree(that); }
public void visitForLoop(JCForLoop that) { visitTree(that); }
public void visitForeachLoop(JCEnhancedForLoop that) { visitTree(that); }
public void visitLabelled(JCLabeledStatement that) { visitTree(that); }
public void visitSwitch(JCSwitch that) { visitTree(that); }
public void visitCase(JCCase that) { visitTree(that); }
public void visitSynchronized(JCSynchronized that) { visitTree(that); }
public void visitTry(JCTry that) { visitTree(that); }
public void visitCatch(JCCatch that) { visitTree(that); }
public void visitConditional(JCConditional that) { visitTree(that); }
public void visitIf(JCIf that) { visitTree(that); }
public void visitExec(JCExpressionStatement that) { visitTree(that); }
public void visitBreak(JCBreak that) { visitTree(that); }
public void visitContinue(JCContinue that) { visitTree(that); }
public void visitReturn(JCReturn that) { visitTree(that); }
public void visitThrow(JCThrow that) { visitTree(that); }
public void visitAssert(JCAssert that) { visitTree(that); }
public void visitApply(JCMethodInvocation that) { visitTree(that); }
public void visitNewClass(JCNewClass that) { visitTree(that); }
public void visitNewArray(JCNewArray that) { visitTree(that); }
public void visitParenthesis(JCParenthesis that) { visitTree(that); }
public void visitAssign(JCAssignment that) { visitTree(that); }
public void visitAssignOperator(JCAssignOperator that) { visitTree(that); }
public void visitUnary(JCUnary that) { visitTree(that); }
public void visitBinary(JCBinary that) { visitTree(that); }
public void visitTypeCast(JCTypeCast that) { visitTree(that); }
public void visitTypeTest(JCInstanceOf that) { visitTree(that); }
public void visitIndexed(JCArrayAccess that) { visitTree(that); }
public void visitSelect(JCFieldAccess that) { visitTree(that); }
public void visitIdentifier(JCIdentifier that) { visitTree(that); }
public void visitLiteral(JCLiteral that) { visitTree(that); }
public void visitTypeIdentifier(JCPrimitiveTypeTree that) { visitTree(that); }
public void visitTypeArray(JCArrayTypeTree that) { visitTree(that); }
public void visitTypeApply(JCTypeApply that) { visitTree(that); }
public void visitTypeUnion(JCTypeUnion that) { visitTree(that); }
public void visitTypeParameter(JCTypeParameter that) { visitTree(that); }
public void visitWildcard(JCWildcard that) { visitTree(that); }
public void visitTypeBoundKind(TypeBoundKind that) { visitTree(that); }
public void visitAnnotation(JCAnnotation that) { visitTree(that); }
public void visitModifiers(JCModifiers that) { visitTree(that); }
public void visitErroneous(JCErroneous that) { visitTree(that); }
public void visitLetExpr(LetExpression that) { visitTree(that); }
public void visitTree(JCTree that) { Assert.error(); }
}
在从Java源代码生成class类的过程中，主要经历的过程
其中实现这个接口的类有：
（1）Enter
visitTree(JCTree)方法是一个空实现，说明了Enter类只处理JCCompilationUnit，JCClassDeclaration和TypeParameter语法节点。
（2）MemberEnter
visitTree(JCTree)方法同样是一个空实现 ，主要处理的语法节点为JCCompilationUnit，JCImport，JCMethodDeclaration与JCVariableDeclaration语法节点。
（3）Attr
在Attr中开始对各个语法节点进行了标记。
（4）TreeScanner（Flow继承实现了 TreeScanner）
这个类就是对各个语法节点进行简单的遍历实现，遍历某个语法节点下的方法如下：
/** Visitor method: Scan a single node.
*/
public void scan(JCTree tree) {
if(tree!=null){
tree.accept(this);
}
}
/** Visitor method: scan a list of nodes.
*/
public void scan(List<? extends JCTree> trees) {
if (trees != null){
for (List<? extends JCTree> l = trees; l.nonEmpty(); l = l.tail){
scan(l.head);
}
}
}
继承这个类进行实现的一些类主要有Flow，还有一些内部类和匿名类。
（5）TreeTranslator
直接继承了这个类的有
1、TranslateTypes This pass translates Generic Java to conventional Java
2、Lower This pass translates away some syntactic sugar: inner classes,class literals, assertions, foreach loops, etc.
主要的实现如下：
public class TreeTranslator extends JCTree.Visitor {
/** Visitor result field: a tree
*/
protected JCTree result;
/** Visitor method: Translate a single node.
*/
@SuppressWarnings("unchecked")
public <T extends JCTree> T translate(T tree) {
if (tree == null) {
return null;
} else {
tree.accept(this);
JCTree result = this.result;
this.result = null;
return (T)result; // XXX cast
}
}
/** Visitor method: translate a list of nodes.
*/
public <T extends JCTree> List<T> translate(List<T> trees) {
if (trees == null){
return null;
}
for (List<T> l = trees; l.nonEmpty(); l = l.tail) {
l.head = translate(l.head);
}
return trees;
}
/** Visitor method: translate a list of variable definitions.
*/
public List<JCVariableDeclaration> translateVarDefs(List<JCVariableDeclaration> trees) {
for (List<JCVariableDeclaration> l = trees; l.nonEmpty(); l = l.tail)
l.head = translate(l.head);
return trees;
}
/** Visitor method: translate a list of type parameters.
*/
public List<JCTypeParameter> translateTypeParams(List<JCTypeParameter> trees) {
for (List<JCTypeParameter> l = trees; l.nonEmpty(); l = l.tail)
l.head = translate(l.head);
return trees;
}
/** Visitor method: translate a list of case parts of switch statements.
*/
public List<JCCase> translateCases(List<JCCase> trees) {
for (List<JCCase> l = trees; l.nonEmpty(); l = l.tail)
l.head = translate(l.head);
return trees;
}
/** Visitor method: translate a list of catch clauses in try statements.
*/
public List<JCCatch> translateCatchers(List<JCCatch> trees) {
for (List<JCCatch> l = trees; l.nonEmpty(); l = l.tail)
l.head = translate(l.head);
return trees;
}
/** Visitor method: translate a list of catch clauses in try statements.
*/
public List<JCAnnotation> translateAnnotations(List<JCAnnotation> trees) {
for (List<JCAnnotation> l = trees; l.nonEmpty(); l = l.tail)
l.head = translate(l.head);
return trees;
}
// Visitor Method省略
}
（6）Gen
生成最终的class类时需要访问的一些语法节点。
（7）Type.Visitor<R, S>, Symbol.Visitor<String, Locale>
类型Type中定义的访问者模式：
/**
* A visitor for types. A visitor is used to implement operations
* (or relations) on types. Most common operations on types are
* binary relations and this interface is designed for binary
* relations, that is, operations on the form
* Type × S → R.
* <!-- In plain text: Type x S -> R -->
*
* @param <R> the return type of the operation implemented by this
* visitor; use Void if no return type is needed.
* @param <S> the type of the second argument (the first being the
* type itself) of the operation implemented by this visitor; use
* Void if a second argument is not needed.
*/
public interface Visitor<R,S> {
R visitClassType(ClassType t, S s);
R visitWildcardType(WildcardType t, S s);
R visitArrayType(ArrayType t, S s);
R visitMethodType(MethodType t, S s);
R visitPackageType(PackageType t, S s);
R visitTypeVar(TypeVar t, S s);
R visitCapturedType(CapturedType t, S s);
R visitForAll(ForAll t, S s);
R visitUndeterminedVar(UndeterminedVar t, S s);
R visitErrorType(ErrorType t, S s);
R visitType(Type t, S s);
}
符号类中定义的访问者模式接口如下：
/**
* A visitor for symbols. A visitor is used to implement operations
* (or relations) on symbols. Most common operations on types are
* binary relations and this interface is designed for binary
* relations, that is, operations on the form
* Symbol × P → R.
* <!-- In plain text: Type x P -> R -->
*
* @param <R> the return type of the operation implemented by this
* visitor; use Void if no return type is needed.
* @param <P> the type of the second argument (the first being the
* symbol itself) of the operation implemented by this visitor; use
* Void if a second argument is not needed.
*/
public interface Visitor<R,P> {
R visitClassSymbol(ClassSymbol s, P arg);
R visitMethodSymbol(MethodSymbol s, P arg);
R visitPackageSymbol(PackageSymbol s, P arg);
R visitOperatorSymbol(OperatorSymbol s, P arg);
R visitVarSymbol(VarSymbol s, P arg);
R visitTypeSymbol(TypeSymbol s, P arg);
R visitSymbol(Symbol s, P arg);
}
（8）TreePretty
剩下的（5）和（6）对定义出的visitorXXX()方法都有实现。