BAEL-634 javassist (#1349)

* BEEL-634 javassist dependency

* BEEL-634 code for javassist article

* BEEL-634 test refinement

* BEEL-634 increment lib to newest version

* add test that uses reflection to verify

* add field

* add bytecode to different class

libraries/pom.xml

@@ -23,7 +23,7 @@
 		</plugins>
 	</build>
 
-	<dependencies>
+  <dependencies>
 		<!-- https://mvnrepository.com/artifact/cglib/cglib -->
 		<dependency>
 			<groupId>cglib</groupId>
@@ -51,6 +51,11 @@
 			<artifactId>javatuples</artifactId>
 			<version>${javatuples.version}</version>
 		</dependency>
+    <dependency>
+        <groupId>org.javassist</groupId>
+        <artifactId>javassist</artifactId>
+        <version>${javaassist.version}</version>
+    </dependency>
 		<!-- https://mvnrepository.com/artifact/org.assertj/assertj-core -->
 		<dependency>
 			<groupId>org.assertj</groupId>
@@ -65,6 +70,7 @@
 		<junit.version>4.12</junit.version>
 		<jasypt.version>1.9.2</jasypt.version>
 		<javatuples.version>1.2</javatuples.version>
+    <javaassist.version>3.21.0-GA</javaassist.version>
 		<assertj.version>3.6.2</assertj.version>
 	</properties>
 

libraries/src/main/java/com/baeldung/javasisst/Point.java

@@ -0,0 +1,17 @@
+package com.baeldung.javasisst;
+
+
+public class Point {
+    public int x = 0;
+    public int y = 0;
+
+    public Point(int x, int y) {
+        this.x = x;
+        this.y = y;
+    }
+
+    public void move(int x, int y) {
+        this.x = x;
+        this.y = y;
+    }
+}

libraries/src/main/java/com/baeldung/javasisst/ThreeDimensionalPoint.java

@@ -0,0 +1,19 @@
+package com.baeldung.javasisst;
+
+
+public class ThreeDimensionalPoint {
+    public int x = 0;
+    public int y = 0;
+    public int z = 0;
+
+    public ThreeDimensionalPoint(int x, int y, int z) {
+        this.x = x;
+        this.y = y;
+        this.z = z;
+    }
+
+    public void move(int x, int y) {
+        this.x = x;
+        this.y = y;
+    }
+}

libraries/src/test/java/com/baeldung/javassist/JavasisstTest.java

@@ -0,0 +1,119 @@
+package com.baeldung.javassist;
+
+
+import javassist.CannotCompileException;
+import javassist.ClassPool;
+import javassist.NotFoundException;
+import javassist.bytecode.*;
+import org.junit.Test;
+
+import java.io.DataOutputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.lang.reflect.Field;
+import java.nio.file.Files;
+import java.nio.file.Paths;
+import java.util.Arrays;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+public class JavasisstTest {
+    @Test
+    public void givenJavasisstAPI_whenConstructClass_thenGenerateAClassFile() throws CannotCompileException, IOException, ClassNotFoundException, IllegalAccessException, InstantiationException {
+        //given
+        String classNameWithPackage = "com.baeldung.JavassistGeneratedClass";
+        ClassFile cf = new ClassFile(false, classNameWithPackage, null);
+        cf.setInterfaces(new String[]{"java.lang.Cloneable"});
+
+        FieldInfo f = new FieldInfo(cf.getConstPool(), "id", "I");
+        f.setAccessFlags(AccessFlag.PUBLIC);
+        cf.addField(f);
+
+        //when
+        String className = "JavassistGeneratedClass.class";
+        cf.write(new DataOutputStream(new FileOutputStream(className)));
+
+        //then
+        ClassPool classPool = ClassPool.getDefault();
+        Field[] fields = classPool.makeClass(cf).toClass().getFields();
+        assertEquals(fields[0].getName(), "id");
+
+        String classContent = new String(Files.readAllBytes(Paths.get(className)));
+        assertTrue(classContent.contains("java/lang/Cloneable"));
+    }
+
+    @Test
+    public void givenJavaClass_whenLoadAtByJavassist_thenTraversWholeClass() throws NotFoundException, CannotCompileException, BadBytecode {
+        //given
+        ClassPool cp = ClassPool.getDefault();
+        ClassFile cf = cp.get("com.baeldung.javasisst.Point").getClassFile();
+        MethodInfo minfo = cf.getMethod("move");
+        CodeAttribute ca = minfo.getCodeAttribute();
+        CodeIterator ci = ca.iterator();
+
+        //when
+        List<String> operations = new LinkedList<>();
+        while (ci.hasNext()) {
+            int index = ci.next();
+            int op = ci.byteAt(index);
+            operations.add(Mnemonic.OPCODE[op]);
+        }
+
+        //then
+        assertEquals(operations,
+                Arrays.asList("aload_0", "iload_1", "putfield", "aload_0", "iload_2", "putfield", "return"));
+
+    }
+
+    @Test
+    public void givenTableOfInstructions_whenAddNewInstruction_thenShouldConstructProperSequence() throws NotFoundException, BadBytecode, CannotCompileException, IllegalAccessException, InstantiationException {
+        //given
+        ClassFile cf = ClassPool.getDefault().get("com.baeldung.javasisst.ThreeDimensionalPoint").getClassFile();
+
+        //when
+        FieldInfo f = new FieldInfo(cf.getConstPool(), "id", "I");
+        f.setAccessFlags(AccessFlag.PUBLIC);
+        cf.addField(f);
+
+
+        ClassPool classPool = ClassPool.getDefault();
+        Field[] fields = classPool.makeClass(cf).toClass().getFields();
+        List<String> fieldsList = Stream.of(fields).map(Field::getName).collect(Collectors.toList());
+        assertTrue(fieldsList.contains("id"));
+
+    }
+
+    @Test
+    public void givenLoadedClass_whenAddConstructorToClass_shouldCreateClassWithConstructor() throws NotFoundException, CannotCompileException, BadBytecode {
+        //given
+        ClassFile cf = ClassPool.getDefault().get("com.baeldung.javasisst.Point").getClassFile();
+        Bytecode code = new Bytecode(cf.getConstPool());
+        code.addAload(0);
+        code.addInvokespecial("java/lang/Object", MethodInfo.nameInit, "()V");
+        code.addReturn(null);
+
+        //when
+        MethodInfo minfo = new MethodInfo(cf.getConstPool(), MethodInfo.nameInit, "()V");
+        minfo.setCodeAttribute(code.toCodeAttribute());
+        cf.addMethod(minfo);
+
+        //then
+        CodeIterator ci = code.toCodeAttribute().iterator();
+        List<String> operations = new LinkedList<>();
+        while (ci.hasNext()) {
+            int index = ci.next();
+            int op = ci.byteAt(index);
+            operations.add(Mnemonic.OPCODE[op]);
+        }
+
+        assertEquals(operations,
+                Arrays.asList("aload_0", "invokespecial", "return"));
+
+
+    }
+}