一、说明
符号表是由一组符号地址和符号信息构成的表格。符号表中所登记的信息在编译的不同阶段都要用到，在语义分析（后面的步骤）中，符号表所登记的内容将用于语义检查和产生中间代码，在目标代码生成阶段，党对符号名进行地址分配时，符号表是地址分配的依据。二、主要的类与方法
解析和填充符号表这个过程主要由com.sun.tools.javac.comp.Entry及com.sun.tools.javac.comp.MemberEnter两个类来实现的。
com.sun.tools.javac.comp.Entry 主要的方法如下：
/**
* 访问类声明
*/
public void visitClassDef(JCClassDecl tree) {
Symbol owner = env.info.scope.owner;
Scope enclScope = enterScope(env);
ClassSymbol c;
if (owner.kind == PCK) {
// We are seeing a toplevel class.
PackageSymbol packge = (PackageSymbol)owner;
for (Symbol q = packge; q != null && q.kind == PCK; q = q.owner)
q.flags_field |= EXISTS;
c = reader.enterClass(tree.name, packge);
packge.members().enterIfAbsent(c);
if ((tree.mods.flags & PUBLIC) != 0 && !classNameMatchesFileName(c, env)) {
log.error(tree.pos(),
"class.public.should.be.in.file", tree.name);
}
} else {
if (tree.name.len != 0 &&
!chk.checkUniqueClassName(tree.pos(), tree.name, enclScope)) {
result = null;
return;
}
if (owner.kind == TYP) {
// We are seeing a member class.
c = reader.enterClass(tree.name, (TypeSymbol)owner);
if ((owner.flags_field & INTERFACE) != 0) {
tree.mods.flags |= PUBLIC | STATIC;
}
} else {
// We are seeing a local class.
c = reader.defineClass(tree.name, owner);
c.flatname = chk.localClassName(c);
if (c.name.len != 0)
chk.checkTransparentClass(tree.pos(), c, env.info.scope);
}
}
tree.sym = c;
// Enter class into `compiled' table and enclosing scope.
if (chk.compiled.get(c.flatname) != null) {
duplicateClass(tree.pos(), c);
result = new ErrorType(tree.name, (TypeSymbol)owner);
tree.sym = (ClassSymbol)result.tsym;
return;
}
chk.compiled.put(c.flatname, c);
enclScope.enter(c);
// Set up an environment for class block and store in `typeEnvs'
// table, to be retrieved later in memberEnter and attribution.
Env<AttrContext> localEnv = classEnv(tree, env);
typeEnvs.put(c, localEnv);
// Fill out class fields.
c.completer = memberEnter;
c.flags_field = chk.checkFlags(tree.pos(), tree.mods.flags, c, tree);
c.sourcefile = env.toplevel.sourcefile;
c.members_field = new Scope(c);
ClassType ct = (ClassType)c.type;
if (owner.kind != PCK && (c.flags_field & STATIC) == 0) {
// We are seeing a local or inner class.
// Set outer_field of this class to closest enclosing class
// which contains this class in a non-static context
// (its "enclosing instance class"), provided such a class exists.
Symbol owner1 = owner;
while ((owner1.kind & (VAR | MTH)) != 0 &&
(owner1.flags_field & STATIC) == 0) {
owner1 = owner1.owner;
}
if (owner1.kind == TYP) {
ct.setEnclosingType(owner1.type);
}
}
// Enter type parameters.
ct.typarams_field = classEnter(tree.typarams, localEnv);
// Add non-local class to uncompleted, to make sure it will be
// completed later.
if (!c.isLocal() && uncompleted != null) uncompleted.append(c);
// System.err.println("entering " + c.fullname + " in " + c.owner);//DEBUG
// Recursively enter all member classes.
classEnter(tree.defs, localEnv);
result = c.type;
}
/** Main method: enter all classes in a list of toplevel trees.
* @param trees The list of trees to be processed.
*/
public void main(List<JCCompilationUnit> trees) {
complete(trees, null);
}
/**
* Main method: enter one class from a list of toplevel trees and
* place the rest on uncompleted for later processing.
* @param trees The list of trees to be processed.
* @param c The class symbol to be processed.
*/
public void complete(List<JCCompilationUnit> trees, ClassSymbol c) {
annotate.enterStart();
ListBuffer<ClassSymbol> prevUncompleted = uncompleted;
if (memberEnter.completionEnabled) uncompleted = new ListBuffer<ClassSymbol>();
try {
// enter all classes, and construct uncompleted list
classEnter(trees, null);
// complete all uncompleted classes in memberEnter
if (memberEnter.completionEnabled) {
while (uncompleted.nonEmpty()) {
ClassSymbol clazz = uncompleted.next();
if (c == null || c == clazz || prevUncompleted == null)
clazz.complete();
else
//将类符号放入prevUncompleted列表(uncompleted列表)
prevUncompleted.append(clazz);
}
// if there remain any unimported toplevels (these must have
// no classes at all), process their import statements as well.
/**
* uncompleted列表没有的符号(除类符号外)，根据improt声明，给*抽象树都添加了一个MemberEnter对象
* 这些符号(包括类的参数类型符号也就是泛型、父类符号、接口类型符等)
*/
for (JCCompilationUnit tree : trees) {
if (tree.starImportScope.elems == null) {
JavaFileObject prev = log.useSource(tree.sourcefile);
Env<AttrContext> env = typeEnvs.get(tree);
if (env == null)
env = topLevelEnv(tree);
memberEnter.memberEnter(tree, env);
log.useSource(prev);
}
}
}
} finally {
//prevUncompleted列表赋值给uncompleted列表
uncompleted = prevUncompleted;
annotate.enterDone();
}
}
com.sun.tools.javac.comp.MemberEnter 主要的方法如下:
/** Complete entering a class.
* 将未处理列表中的所有符号都解析到各自的类符号表中
* @param sym The symbol of the class to be completed.
*/
public void complete(Symbol sym) throws CompletionFailure {
// Suppress some (recursive) MemberEnter invocations
if (!completionEnabled) {
// Re-install same completer for next time around and return.
assert (sym.flags() & Flags.COMPOUND) == 0;
sym.completer = this;
return;
}
ClassSymbol c = (ClassSymbol)sym;
ClassType ct = (ClassType)c.type;
Env<AttrContext> env = enter.typeEnvs.get(c);
JCClassDecl tree = (JCClassDecl)env.tree;
boolean wasFirst = isFirst;
isFirst = false;
JavaFileObject prev = log.useSource(env.toplevel.sourcefile);
try {
// Save class environment for later member enter (2) processing.
halfcompleted.append(env);
// If this is a toplevel-class, make sure any preceding import
// clauses have been seen.
if (c.owner.kind == PCK) {
memberEnter(env.toplevel, env.enclosing(JCTree.TOPLEVEL));
todo.append(env);
}
// Mark class as not yet attributed.
c.flags_field |= UNATTRIBUTED;
if (c.owner.kind == TYP)
c.owner.complete();
// create an environment for evaluating the base clauses
Env<AttrContext> baseEnv = baseEnv(tree, env);
// Determine supertype.
Type supertype =
(tree.extending != null)
? attr.attribBase(tree.extending, baseEnv, true, false, true)
: ((tree.mods.flags & Flags.ENUM) != 0 && !target.compilerBootstrap(c))
? attr.attribBase(enumBase(tree.pos, c), baseEnv,
true, false, false)
: (c.fullname == names.java_lang_Object)
? Type.noType
: syms.objectType;
ct.supertype_field = supertype;
// Determine interfaces.
ListBuffer<Type> interfaces = new ListBuffer<Type>();
Set<Type> interfaceSet = new HashSet<Type>();
List<JCExpression> interfaceTrees = tree.implementing;
if ((tree.mods.flags & Flags.ENUM) != 0 && target.compilerBootstrap(c)) {
// add interface Comparable<T>
interfaceTrees =
interfaceTrees.prepend(make.Type(new ClassType(syms.comparableType.getEnclosingType(),
List.of(c.type),
syms.comparableType.tsym)));
// add interface Serializable
interfaceTrees =
interfaceTrees.prepend(make.Type(syms.serializableType));
}
for (JCExpression iface : interfaceTrees) {
Type i = attr.attribBase(iface, baseEnv, false, true, true);
if (i.tag == CLASS) {
interfaces.append(i);
chk.checkNotRepeated(iface.pos(), types.erasure(i), interfaceSet);
}
}
if ((c.flags_field & ANNOTATION) != 0)
ct.interfaces_field = List.of(syms.annotationType);
else
ct.interfaces_field = interfaces.toList();
if (c.fullname == names.java_lang_Object) {
if (tree.extending != null) {
chk.checkNonCyclic(tree.extending.pos(),
supertype);
ct.supertype_field = Type.noType;
}
else if (tree.implementing.nonEmpty()) {
chk.checkNonCyclic(tree.implementing.head.pos(),
ct.interfaces_field.head);
ct.interfaces_field = List.nil();
}
}
// Annotations.
// In general, we cannot fully process annotations yet, but we
// can attribute the annotation types and then check to see if the
// @Deprecated annotation is present.
attr.attribAnnotationTypes(tree.mods.annotations, baseEnv);
if (hasDeprecatedAnnotation(tree.mods.annotations))
c.flags_field |= DEPRECATED;
annotateLater(tree.mods.annotations, baseEnv, c);
attr.attribTypeVariables(tree.typarams, baseEnv);
chk.checkNonCyclic(tree.pos(), c.type);
/**
* 增加一个默认的构造方法(当类没有构造方法时)
*/
if ((c.flags() & INTERFACE) == 0 &&
!TreeInfo.hasConstructors(tree.defs)) {
List<Type> argtypes = List.nil();
List<Type> typarams = List.nil();
List<Type> thrown = List.nil();
long ctorFlags = 0;
boolean based = false;
if (c.name.len == 0) {
JCNewClass nc = (JCNewClass)env.next.tree;
if (nc.constructor != null) {
Type superConstrType = types.memberType(c.type,
nc.constructor);
argtypes = superConstrType.getParameterTypes();
typarams = superConstrType.getTypeArguments();
ctorFlags = nc.constructor.flags() & VARARGS;
if (nc.encl != null) {
argtypes = argtypes.prepend(nc.encl.type);
based = true;
}
thrown = superConstrType.getThrownTypes();
}
}
JCTree constrDef = DefaultConstructor(make.at(tree.pos), c,
typarams, argtypes, thrown,
ctorFlags, based);
tree.defs = tree.defs.prepend(constrDef);
}
// If this is a class, enter symbols for this and super into
// current scope.
if ((c.flags_field & INTERFACE) == 0) {
VarSymbol thisSym =
new VarSymbol(FINAL | HASINIT, names._this, c.type, c);
thisSym.pos = Position.FIRSTPOS;
env.info.scope.enter(thisSym);
if (ct.supertype_field.tag == CLASS) {
VarSymbol superSym =
new VarSymbol(FINAL | HASINIT, names._super,
ct.supertype_field, c);
superSym.pos = Position.FIRSTPOS;
env.info.scope.enter(superSym);
}
}
// check that no package exists with same fully qualified name,
// but admit classes in the unnamed package which have the same
// name as a top-level package.
if (check* &&
c.owner.kind == PCK && c.owner != syms.unnamedPackage &&
reader.packageExists(c.fullname))
{
log.error(tree.pos, "*.with.pkg.of.same.name", c);
}
} catch (CompletionFailure ex) {
chk.completionError(tree.pos(), ex);
} finally {
log.useSource(prev);
}
// Enter all member fields and methods of a set of half completed
// classes in a second phase.
if (wasFirst) {
try {
while (halfcompleted.nonEmpty()) {
finish(halfcompleted.next());
}
} finally {
isFirst = true;
}
// commit pending annotations
annotate.flush();
}
}
三、过程及简单源码解析
Enter过程中，编译器会找到当前范围(enclosing scope)中发现的所有的定义(definitions)，并且把这些定义注册成符号（symbols）。Enter又分为以下两个阶段：
第一个阶段:
编译器会注册所有类的符号，并且把这写符号和相应的范围（scope）联系在一起。实现方法是使用一个Visitor（访问者）类，由上而下的遍历AST(抽象语法树)，访问所有的类，包括类里面的内部类。Enter给每一个类的符号都添加了一个MemberEnter对象，这个对象是由第二个阶段来调用的。
整个操作的方法调用过程如下：
上面这个过程是访问者模式的一种实现。
Enter是一个JCTree.Visitor.Enter.classEnter(l.head, env)调用JCTree.accept(Visitor v)，而accept方法又是调用的Visitor类里面的visitXXX()方法，而这些方法的实现又是在Enter类中。也就是Enter.visitClassDef(JCClassDecl tree)方法，在这个方法中，会将类符号放入uncompleted列表;
visitClassDef(JCClassDecl tree)方法主要做三件事：
1、将类符号(当前类)填入类自身的符号表，添加了一个MemberEnter对象
// Enter class into `compiled' table and enclosing scope.
if (chk.compiled.get(c.flatname) != null) {
duplicateClass(tree.pos(), c);
result = new ErrorType(tree.name, (TypeSymbol)owner);
tree.sym = (ClassSymbol)result.tsym;
return;
}
chk.compiled.put(c.flatname, c);
enclScope.enter(c);
// Set up an environment for class block and store in `typeEnvs'
// table, to be retrieved later in memberEnter and attribution.
Env<AttrContext> localEnv = classEnv(tree, env);
typeEnvs.put(c, localEnv);
// Fill out class fields.
c.completer = memberEnter;
c.flags_field = chk.checkFlags(tree.pos(), tree.mods.flags, c, tree);
c.sourcefile = env.toplevel.sourcefile;
c.members_field = new Scope(c);
2、解析填写其它的类符号,包括当前类中使用到的内部类、枚举、变量等抽象树的类符号。
// Enter type parameters.
ct.typarams_field = classEnter(tree.typarams, localEnv);
3、将类符号放入uncompleted列表
if (!c.isLocal() && uncompleted != null) uncompleted.append(c);
Enter给每一个类的符号都添加了一个MemberEnter对象，这个对象是由第二个阶段来调用的。
memberEnter.memberEnter(tree, env);
第二个阶段:
这些类被MemberEnter对象所完成（completed，即完成类的成员变量的Enter）。首先，MemberEnter决定一个类的参数，父类和接口。然后这些符号被添加进了类的范围中。不像前一个步骤，这个步骤是懒惰执行的。类的成员只有在被访问时，才加入类的定义中的。这里的实现，是通过安装一个完成对象（member object）到类的符号中。这些对象可以在需要时调用memberEnter。
整个操作的方法调用过程如下：
Enter是一个JCTree.Visitor.Enter.classEnter(l.head, env)调用JCTree.accept(Visitor v)，而accept方法又是调用的Visitor类里面的visitXXX()方法，而这些方法的实现又是在Enter类中。也就是Enter.visitClassDef(JCClassDecl tree)方法，在这个方法中，会将类符号解析和填入类自身的符号表。
最后，enter把所有的顶层类（top-level classes）放到一个todo-queue中。