Visualizing the Spoon AST of a Java file

To get started with the metamodel, you can browse the model (ie the Spoon AST) of a java file (say as follows:

$ java -cp spoon-core-8.1.0-jar-with-dependencies.jar spoon.Launcher -i --gui

If you have Java 11 with Java FX, there is a new GUI, see

Your first processor

In Spoon, a processor is a combination of query and analysis code. With this concept, developer can analyse all elements of a type given and inspect each element of this type one per one.

For a first processor, we'll analyze all catch blocks of a try {...} catch {...} element to know how many empty catch blocks we have in a project. This kind of empty catch can be considered bad practice. That could be a great information to know how many and where are these catches in a project to fill them with some code, e.g. throws a runtime exception or logs the exception.

// file processors/
package processors;

import org.apache.log4j.Level;
import spoon.processing.AbstractProcessor;
import spoon.reflect.code.CtCatch;

 * Reports warnings when empty catch blocks are found.
public class CatchProcessor extends AbstractProcessor<CtCatch> {
    public void process(CtCatch element) {
        // we get all statements and if there isn't statement, it means the block catch is empty!
        if (element.getBody().getStatements().size() == 0) {
            getFactory().getEnvironment().report(this, Level.WARN, element, "empty catch clause");

This processor extends AbstractProcessor (javadoc). This super class takes a generic type parameter to know what type you want inspect in a AST. For this tutorial, we inspect a catch, a CtCatch (javadoc).

When the class AbstractProcessor (javadoc) is extended, we implement the method void process(E element)where E is a generic type for any elements of the AST (all classes in the Spoon meta model which extends CtElement (javadoc)). It is in this method that you can access all information you want of the the current CtCatch (javadoc).

Apply the processor

First, compile your processor. You can use javac in command line to generate the CatchProcessor.class file. Then we execute Spoon as follows to analyze all catch blocks in Java files that are in /path/to/src/of/your/project:

$ java -classpath /path/to/binary/of/your/processor.jar:spoon-core-8.1.0-jar-with-dependencies.jar spoon.Launcher -i /path/to/src/of/your/project -p processors.CatchProcessor

There are many more examples of source code analysis in

Bytecode analysis

Note that spoon also supports the analysis of bytecode through decompilation. See javadoc.