The Abstract Syntax Tree is the base framework for many powerful tools of the Eclipse IDE, including refactoring, Quick Fix and Quick Assist. The Abstract Syntax Tree maps plain Java source code in a tree form. This tree is more convenient and reliable to analyse and modify programmatically than text-based source. This article shows how you can use the Abstract Syntax Tree for your own applications.
Are you wondering how Eclipse is doing all the magic like jumping conveniently to a declaration, when you press "F3" on a reference to a field or method? Or how "Replace in file" solidly detects the declaration and all the references to the local variable and modifies them synchronously?
Well, these—and a big portion of the other source code modification and generation tools—are based upon the Abstract Syntax Tree (AST). The AST is comparable to the DOM tree model of an XML file. Just like with DOM, the AST allows you to modify the tree model and reflects these modifications in the Java source code.
This article refers to an example application which covers most of the interesting AST-related topics. Let us have a look at the application that was built to illustrate this article:
According to Java Practices [4], you should not declare local variables before using them. The goal of our application will be to detect contradicting variable declarations and to move them to their correct place. There are three cases our application has to deal with:
Removal of unnecessary declaration. If a variable is declared and initialized, only to be overridden by another assignment later on, the first declaration of the variable is an unnecessary declaration.
Move of declaration. If a variable is declared, and not immediately referenced within the following statement, this variable declaration has to be moved. The correct place for the declaration is the line before it is first referenced.
Move of declaration of a variable that is referred to from within different blocks. This is a subcase of case 2. Imagine that a variable is used in both a try- and a catch clause. Here the declaration cannot be moved right before the first reference in the try-clause, since then it would not be declared in the catch-clause. Our application has to deal with that and has to move the declaration to the best possible place, which would be here one line above the try-clause.
You can import the example application into your workspace [1] or install the plug-in using the Eclipse Update Manager [2].
A typical workflow of an application using AST looks like this:
char[]
that contains Java sourceorg.eclipse.jdt.core.dom.ASTParser
. See the section called “Parsing source code”.Manipulating the AST: If the AST of point 3 needs to be changed, this can be done in two ways:
ASTRewrite
.IDocument
: Is a wrapper for the source code of step
1 and is needed at point 5 As mentioned, the Abstract Syntax Tree is the way that Eclipse looks at your source
code: every Java source file is entirely represented as tree of AST nodes. These nodes
are all subclasses of ASTNode
. Every subclass is
specialized for an element of the Java Programming Language. E.g. there are nodes for
method declarations ( MethodDeclaration
), variable
declaration ( VariableDeclarationFragment
),
assignments and so on. One very frequently used node is
SimpleName
. A SimpleName
is any
string of Java source that is not a keyword, a Boolean literal (
true
or
false
) or the
null
literal. For example, in
i = 6 + j;
,
i
and
j
are represented by SimpleName
s. In
import net.sourceforge.earticleast
,
net
sourceforge
and
earticleast
are mapped to SimpleName
s.
All AST-relevant classes are located in the package
org.eclipse.jdt.core.dom
of the
org.eclipse.jdt.core
plug-in.
To discover how code is represented as AST, the AST Viewer plug-in [5] is a big help: Once installed you can simply mark source code in the editor and let it be displayed in a tree form in the AST Viewer view.
Most of the time, an AST is not created from scratch, but rather parsed from
existing Java code. This is done using the ASTParser
. It
processes whole Java files as well as portions of Java code. In the example
application the method parse(ICompilationUnit unit)
of the class AbstractASTArticle
parses the source code
stored in the file that
unit
points to:
protected CompilationUnit parse(ICompilationUnit unit) { ASTParser parser = ASTParser.newParser(AST.JLS3); parser.setKind(ASTParser.K_COMPILATION_UNIT); parser.setSource(unit); // set source parser.setResolveBindings(true); // we need bindings later on return (CompilationUnit) parser.createAST(null /* IProgressMonitor */); // parse }
With
ASTParser.newParser(AST.JLS3)
, we advise
the parser to parse the code following to the Java Language Specification, Third
Edition. JLS3 includes all Java Language Specifications up to the new syntax
introduced in Java 5. With the update of Eclipse towards JLS3, changes have been made
to the AST API. To preserve compatibility, the ASTParser
can be run in the deprecated JLS2 mode.
parser.setKind(ASTParser.K_COMPILATION_UNIT)
tells the parser,
that it has to expect an ICompilationUnit
as input. An
ICompilationUnit
is a pointer to a Java file. The parser
supports five kinds of input:
Entire source file: The parser expects the source
either as a pointer to a Java file (which means as an
ICompilationUnit
, see the section called “Java Model”) or as
char[]
.
K_COMPILATION_UNIT
Portion of Java code: The parser processes a portion of
code. The input format is
char[]
.
K_EXPRESSION
: the input contains a Java expression. E.g.
new String()
,
4+6
or
i
.K_STATEMENTS
: the input contains a Java statement like
new String();
or
synchronized (this) { ... }
.K_CLASS_BODY_DECLARATIONS
: the input contains elements of a
Java class like method declarations, field declarations, static blocks,
etc.The Java Model is a whole different story. It is out of scope of this article to dive deep into its details within. The parts looked at will be the ones which intersect with the AST. The motivation to discuss it here is, to use it as an entry point to build an Abstract Syntax Tree of a source file. Remember, the
ICompilationUnit
is one of the possible parameters for
the AST parser.
The Java Model represents a Java Project in a tree structure, which is visualized by the well known "Package Explorer" view:
The nodes of the Java Model implement one of the following interfaces:
IJavaProject
: Is the node of the Java Model and represents a
Java Project. It contains
IPackageFragmentRoot
s as child nodes.IPackageFragmentRoot
: can be a source or a class folder of a
project, a
.zip
or a
.jar
file.
IPackageFragmentRoot
can hold source or binary
files.IPackageFragment
: A single package. It contains
ICompilationUnit
s or
IClassFile
s, depending on whether the
IPackageFragmentRoot
is of type source or of type binary.
Note that
IPackageFragment
are not organized as parent-children.
E.g.
net.sf.a
is not the parent of
net.sf.a.b
. They are two independent children of the same
IPackageFragmentRoot
.ICompilationUnit
: a Java source file. IImportDeclaration
,
IType
,
IField
,
IInitializer
,
IMethod
: children of
ICompilationUnit
. The information provided by these nodes
is available from the AST, too. In contrast to the AST, these nodes are lightweight handles. It costs much less
to rebuild a portion of the Java Model than to rebuild an AST. That is also one reason
why the Java Model is not only defined down to the level of
ICompilationUnit
. There are many cases where complete
information, like that provided by the AST, is not needed. One example is the Outline
view: this view does not need to know the contents of a method body. It is more
important that it can be rebuilt fast, to keep in sync with its source code.
There are different ways to get an
ICompilationUnit
. The example applications are
launched as actions from the package tree view. This is quite convenient: only add
an
objectContribution
extension to the point
org.eclipse.ui.popupMenus
. By choosing
org.eclipse.jdt.core.ICompilationUnit
as
objectClass
, the action will be only displayed in the context menu
of a compilation unit. Have a look at the example application's
plugin.xml
. The compilation unit then can be retrieved from the
ISelection
, that is passed to the
action's delegate (in the example, this is
ASTArticleActionDelegate
).
Another, programmatic, approach is to get the project handle from the IDE and to
look for the compilation unit. This can be done by either step down the Java Model
tree to collect the desired ICompilationUnit
s. Or, if
the qualified name of a type within the compilation unit is known, by calling the
findType()
of the Java project:
IWorkspaceRoot root = ResourcesPlugin.getWorkspace().getRoot(); IProject project = root.getProject("someJavaProject"); project.open(null /* IProgressMonitor */); IJavaProject javaProject = JavaCore.create(project); IType lwType = javaProject.findType("net.sourceforge.earticleast.app.Activator"); ICompilationUnit lwCompilationUnit = lwType.getCompilationUnit();
Even a simple "Hello world" program results in a quite complex tree.
How does one get the MethodInvocation
of that
println("Hello World")
? Scanning all the levels is a
possible, but not very convenient.
There is a better solution: every
ASTNode
allows querying for a child node by using a visitor (visitor
pattern [6]). Have a look at
ASTVisitor
. There you'll find for every subclass of
ASTNode
two methods, one called
visit()
, the other called
endVisit()
. Further, the
ASTVisitor
declares these two methods:
preVisit(ASTNode node)
and
postVisit(ASTNode node)
.
The subclass of ASTVisitor
is passed to any node of the
AST. The AST will recursively step through the tree, calling the mentioned methods of
the visitor for every AST node in this order (for the example of a
MethodInvocation
):
preVisit(ASTNode node)
visit(MethodInvocation node)
endVisit(MethodInvocation node)
postVisit(ASTNode node)
In our example application, the
LocalVariableDetector
is a subclass of
ASTVisitor
. It is used, amongst other things, to collect
all local variable declarations of a compilation unit:
public boolean visit(VariableDeclarationStatement node) { for (Iterator iter = node.fragments().iterator(); iter.hasNext();) { VariableDeclarationFragment fragment = (VariableDeclarationFragment) iter.next(); // ... store these fragments somewhere } return false; // prevent that SimpleName is interpreted as reference }
If
false
is returned from visit()
, the
subtree of the visited node will not be considered. This is to ignore parts of
the AST.
In the example, process(CompilationUnit unit)
is
called from the outside to start visiting the compilation unit. The method is fairly
simple:
public void process(CompilationUnit unit) { unit.accept(this); }
Every subclass of
ASTNode
contains specific information for the Java element it
represents. E.g. a
MethodDeclaration
will contain information about the name, return
type, parameters, etc. The information of a node is referred as
structural properties. Let us have a closer look at the
characteristics of the structural properties. Beneath you see the properties of
this method declaration:
public void start(BundleContext context) throws Exception { super.start(context); }
Access to the values of a node's structural properties can be made using static or generic methods:
static methods: every node offers methods to
access its properties: e.g.
getName()
,
exceptions()
, etc.
generic method: ask for a property value using
the getStructuralProperty(StructuralPropertyDescriptor
property)
method. Every AST subclass defines a set of
StructuralPropertyDescriptor
s, one for every
structural property. The
StructuralPropertyDescriptor
can be accessed
directly on the class to which they belong: e.g.
MethodDeclaration.NAME_PROPERTY
. A list of all available
StructuralPropertyDescriptor
s of a node can be retrieved by
calling the method
structuralPropertiesForType()
on any instance of
ASTNode
.
The structural properties are grouped into three different kinds: properties that hold simple values, properties which contain a single child AST node and properties which contain a list of child AST nodes.
SimplePropertyDescriptor
: The value will be a
String
, a primitive value wrapper for either
Integer
or
Boolean
or a basic AST constant. For a list of all possible value
classes of a simple property, see Appendix C, Simple properties value classes
ChildPropertyDescriptor
: The value will be a node, an
instance of an
ASTNode
subclass
ChildListPropertyDescriptor
: The value will be a
List
of AST nodes
The AST, as far as we know it, is just a tree-form representation of source code.
Every element of the source code is mapped to a node or a subtree. Looking at a reference to a
variable, let's say
i
, is represented by an instance of
SimpleName
with "i" as
IDENTIFIER
property-value. Bindings go one step further: they provide
extended resolved information for several elements of the AST. About the
SimpleName
above they tell us that it is a reference to a local
variable of type int.
Various subclasses of ASTNode
have binding
information. It is retrieved by calling
resolveBinding()
on these classes. There are cases where
more than one binding is available: e.g. the class
MethodInvocation
returns a binding to the method that is
invoked (resolveMethodBinding()
). Furthermore a
binding to the return type of the method
(resolveTypeBinding()
) and information about whether
the method invocation is involved into a boxing
(resolveBoxing()
) or unboxing
(resolveUnboxing()
) is offered.
Since evaluating bindings is costly, the binding service has to be explicitly
requested at parse time. This is done by passing
true
the method
ASTParser.setResolveBindings()
before the source is being parsed.
int i = 7; System.out.println("Hello!"); int x = i * 2;the reference of the variable
i
is represented by a
SimpleName
. Without bindings you would not know nothing more than this:
Bindings provide more information:
Bindings allow you to comfortably find out to which declaration a reference
belongs, as well as to detect whether two elements are references to the same element: if
they are, the bindings returned by reference-nodes and declaration-nodes are
identical. For example, all SimpleNames
that represent a
reference to a local variable
i
return the same instance of
IVariableBinding
from
SimpleName.resolveBindings()
. The declaration node,
VariableDeclarationFragment.resolveBinding()
, returns the same
instance of
IVariableBinding
, too. If there is another declaration of a local
variable
i
(within another method or block), another instance of
IVariableBinding
is returned. Confusions caused by equally named
elements are avoided if bindings are used to identify an element (variable, method,
type, etc.).
The example application uses variable bindings for this purpose: for every declaration, a manager object is created and added to a map. The binding of the declaration figures as key, the created manager as value.
for (Iterator iter = node.fragments().iterator(); iter.hasNext();) { VariableDeclarationFragment fragment = (VariableDeclarationFragment) iter.next(); IVariableBinding binding = fragment.resolveBinding(); VariableBindingManager manager = new VariableBindingManager(fragment); localVariableManagers.put(binding, manager); }Then, if a
SimpleName
is visited, the application checks, whether the binding of
this
SimpleName
occurs in the map. If so, the
SimpleName
is a reference to a local variable.
public boolean visit(SimpleName node) { IBinding binding = node.resolveBinding(); if (localVariableManagers.containsKey(binding)) { VariableBindingManager manager = localVariableManagers.get(binding); manager.variableRefereneced(node); } }
Since Eclipse 3.2, the DOM/AST support has the ability to recover from code with
syntax errors. In order to trigger it, you need to use the
ASTParser#setStatementsRecovery()
. As its name says it, the error
recovery is done at the statement level. This however implies that the code is not
completely messed up as it cannot recover from all kinds of syntax errors. This being
said a missing semi-colon is no longer an problem anymore to retrieve the statements of a
method. Let's take an example:
public static void main(String[] args) { System.out.print("Hello"); System.out.print(", ") System.out.println("World!"); }
With this source code, before Eclipse 3.2, the method body would be empty. Now with
Eclipse 3.2 and its error recovery it is possible to get a
RECOVERED
expression statement that contains the method invocation. Not
only can you have nodes that can be traversed by a visitor, but in some cases it is even possible to
get bindings for the
RECOVERED
statement. So in the example, you would end up with two statements
that have no problems and one RECOVERED
statement.
Any instance of a subclass of ASTNode
can be tagged with some
bits that provide information about the way the node was created. This bits can be retrieved
by calling the method ASTNode#getFlags()
. The whole list of bits
are:
MALFORMED
: indicates node is syntactically malformedORIGINAL
: indicates original node created by ASTParserPROTECT
: indicates node is protected from further modificationRECOVERED
: indicates node or a part of this node is recovered from source that contains a syntax errorSo when traversing a AST tree, you might want to check the flags of the traversed nodes. A
node flagged as
RECOVERED
might not contain the expected nodes.
This section will show how to modify an AST and how to store these modifications back into Java source code.
New AST nodes may have to be created. New nodes are created by using the class
org.eclipse.jdt.core.dom.AST
(here AST
it is the name of an
actual class. Do not confuse with the abbreviation "AST" used within this
article). Have a look at this class: it offers methods to create every AST node type. An
instance of AST
is created when source code is parsed. This
instance can be obtained from every node of the tree by calling the method
getAST()
. The newly created nodes can only be added to the
tree that class AST
was retrieved from.
Often it is convenient to reuse an existing subtree of an AST and maybe just change
some details. AST nodes cannot be re-parented, once connected to an AST, they
cannot be attached to a different place of the tree. Though it is easy to create a copy from
a subtree:
(Expression) ASTNode.copySubtree(ast, manager.getInitializer())
. The parameter
ast
is the target AST
. This instance will be
used to create the new nodes. That allows copying nodes from another
AST
(established by another parser run) into the current
AST
domain.
There are two APIs to track modifications on an AST: either you can directly modify
the tree or you can make use of a separate protocol, managed by an instance of
ASTRewrite
. The latter, using the
ASTRewrite
, is the more sophisticated and preferable way. The changes
are noted by an instance of
ASTRewrite
, the original AST is left untouched. It is possible to create
more than one instance of
ASTRewrite
for the same AST, which means that different change logs can
be set up. "Quick Fix" makes use of this API: this is how for every Quick Fix
proposal a preview is created.
Example 1. Protocolling changes to a AST by using ASTRewrite
.
rewrite = ASTRewrite.create(unit.getAST()); // unit instance of CompilationUnit // ... VariableDeclarationStatement statement = createNewVariableDeclarationStatement(manager, ast); int firstReferenceIndex = getFirstReferenceListIndex(manager, block); ListRewrite statementsListRewrite = rewrite.getListRewrite(block, Block.STATEMENTS_PROPERTY); statementsListRewrite.insertAt(statement, firstReferenceIndex, null);
MethodInvocation
, the code would look like this:
rewrite.set(methodInvocation, MethodInvocation.NAME_PROPERTY, newName, null);or
rewrite.replace(methodInvocation.getName() /* old name node*/, newName, null)To set a simple property value, call
set()
like shown
above.Let us have a look at the second way to change an AST. Instead of tracking the
modifications in separate protocols, we directly modify the AST. The only thing that
has to done before modifying the first node is to turn on the change recording by calling
recordModifications()
on the root of the AST, the
CompilationUnit
. Internally changes are logged to an
ASTRewrite
as well, but this happens hidden to you.
Example 2. Modifying an AST directly.
unit.recordModifications(); // ... VariableDeclarationStatement statement = createNewVariableDeclarationStatement(manager, ast); block.statements().add(firstReferenceIndex, statement);
The next section will tell how to write the modifications back into Java source code.
Once you have tracked changes, either by using ASTRewrite
or by modifying the tree nodes directly, these changes can be written back into Java
source code. Therefore a TextEdit
object has to be
created. Here we leave the code related area of the AST, and enter a text based
environment. The TextEdit
object contains character
based modification information. It is part of the
org.eclipse.text
plug-in.
How to obtain the
TextEdit
object differs for the two mentioned ways only slightly:
If you used
ASTRewrite
, ask the
ASTRewrite
instance for the desired
TextEdit
object by calling
rewriteAST(IDocument, Map)
.
If you changed the tree nodes directly, the
TextEdit
object is created by calling
rewrite(IDocument document, Map options)
on
CompilationUnit
.
The first parameter,
document
, contains the source code that will be modified. The content of
this container is the same code that you fed into the
ASTParser
. The second parameter is a map of options for the
source code formatter. To use the default options, pass
null
.
Obtaining an
IDocument
if you parsed source code from a
String
is easy: create an object of the class
org.eclipse.jface.text.Document
and pass the code string as
constructor parameter.
If you initially parsed an existing Java source file and would like to store the changes back into this file, things get a little bit more tricky. You should not directly write into this file, since you might not be the only editor that is manipulating this source file. Within Eclipse, Java editors do not write directly on a file resource, but on a shared working copy instead.
ITextFileBufferManager bufferManager = FileBuffers.getTextFileBufferManager(); // get the buffer manager IPath path = unit.getJavaElement().getPath(); // unit: instance of CompilationUnit try { bufferManager.connect(path, null); // (1) ITextFileBuffer textFileBuffer = bufferManager.getTextFileBuffer(path); // retrieve the buffer IDocument document = textFileBuffer.getDocument(); (2) // ... edit the document here ... // commit changes to underlying file textFileBuffer .commit(null /* ProgressMonitor */, false /* Overwrite */); // (3) } finally { bufferManager.disconnect(path, null); // (4) }
path
can be obtained.getTextFileBuffer
. From the
ITextFileBuffer
we get the
IDocument
instance we need.One of the most frustrating part of modifying an AST is the comment handling. The method
CompilationUnit#getCommentList()
is used to return the list of comments
located in the compilation unit in the ascendant order. Unfortunately, this list cannot be
modified. This means that even if the AST Rewriter is used to add a comment inside a
compilation unit, the new comment would not appear inside the comments' list.
In order to add a comment the following code snippet can be used:
CompilationUnit astRoot= ... ; // get the current compilation unit ASTRewrite rewrite= ASTRewrite.create(astRoot.getAST()); Block block= ((TypeDeclaration) astRoot.types().get(0)).getMethods()[0].getBody(); ListRewrite listRewrite= rewrite.getListRewrite(block, Block.STATEMENTS_PROPERTY); Statement placeHolder= rewrite.createStringPlaceholder("//mycomment", ASTNode.EMPTY_STATEMENT); listRewrite.insertFirst(placeHolder, null); textEdits= rewrite.rewriteAST(document, null); textEdits.apply(document);
The methods CompilationUnit#getExtendedLength(ASTNode)
and CompilationUnit#getExtendedStartPosition(ASTNode)
can be used to retrieve the range of a node that would
contains preceding and trailing comments and whitespaces.
If a comment is a javadoc comment defined prior to a method declaration, a field
declaration or a type declaration (including enum, annotations, interfaces and classes),
they can also be retrieved by calling the getJavadoc()
method
on the corresponding declaration.
This article has shown how to use the Eclipse AST for static code analysis and code manipulation issues. It touched the Java Model, explained Bindings and showed how to store changes made to the AST back into Java source code.
For remarks, questions, etc. enter a comment in the bugzilla entry of this article [7].
[1] Download the Packed Example Project. Use the option "Existing Projects into Workspace" from the "Import" Wizard to add it to your workspace.
[2] To install the plug-in, obtain using the Eclipse Update Manager. Update Site: http://earticleast.sourceforge.net/update.
[4] Java Practices .
[5] AST Viewer Plug-in .
In the introduction, three typical cases for our example application have been presented (see the section called “Example Application”). Clarifying code before / after code snippets follow to further clarify these cases.
Removal of unnecessary declaration.
Before:
int x = 0; ... x = 2 * 3;
After:
... int x = 2 * 3;
Move of declaration.
Before:
int x = 0; ... System.out.println(x); ... x = 2 * 3;
After:
... int x = 0; System.out.println(x); ... x = 2 * 3;
Move of a declaration of a variable, that is used within different blocks.
Before:
int x = 0; ... try { x = 2 * 3; } catch (...) { System.out.println(x); }
After:
... int x = 0; try { x = 2 * 3; } catch (...) { System.out.println(x); }
IAnnotationBinding
IMemberValuePairBinding
IMethodBinding
IPackageBinding
ITypeBinding
IVariableBinding
Below the list of all classes of which simple property values can be instance of (in Eclipse version 3.2).
boolean
int
String
Modifier.ModifierKeyword
Assignment.Operator
InfixExpression.Operator
PostfixExpression.Operator
PrefixExpression.Operator
PrimitiveType.Code