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T5750 · Feb 3, 2020 · 11c5805 · 11c5805
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diff --git a/doc/source/jdk8/index.rst b/doc/source/jdk8/index.rst
@@ -8,4 +8,5 @@ Java™ Platform Standard Ed. 8
 
     jdkJre
     Lambda
+    javap
     javassistTutorial
diff --git a/doc/source/jdk8/javap.md b/doc/source/jdk8/javap.md
@@ -0,0 +1,104 @@
+# javap
+
+Disassembles one or more class files.
+
+## Synopsis
+```
+javap [options] classfile...
+```
+- `classfile`: One or more classes separated by spaces to be processed for annotations such as `DocFooter.class`. You can specify a class that can be found in the class path, by its file name or with a URL such as `file:///home/user/myproject/src/DocFooter.class`.
+
+## Description
+The `javap` command disassembles one or more class files. The output depends on the options used. When no options are used, then the `javap` command prints the package, protected and public fields, and methods of the classes passed to it. The `javap` command prints its output to `stdout`.
+
+## Options
+- `-help`, `--help`, `-?`: Prints a help message for the `javap` command.
+- `-version`: Prints release information.
+- `-l`: Prints line and local variable tables.
+- `-public`: Shows only public classes and members.
+- `-protected`: Shows only protected and public classes and members.
+- `-private`, `-p`: Shows all classes and members.
+- `-Joption`: Passes the specified option to the JVM. For example:
+    * `javap -J-version`
+    * `javap -J-Djava.security.manager -J-Djava.security.policy=MyPolicy MyClassName`
+- `-s`: Prints internal type signatures.
+- `-sysinfo`: Shows system information (path, size, date, MD5 hash) of the class being processed.
+- `-constants`: Shows `static final` constants.
+- `-c`: Prints disassembled code, for example, the instructions that comprise the Java bytecodes, for each of the methods in the class.
+- `-verbose`: Prints stack size, number of locals and arguments for methods.
+- `-classpath path`: Specifies the path the `javap` command uses to look up classes. Overrides the default or the `CLASSPATH` environment variable when it is set.
+- `-bootclasspath path`: Specifies the path from which to load bootstrap classes. By default, the bootstrap classes are the classes that implement the core Java platform located in `jre/lib/rt.jar` and several other JAR files.
+- `-extdir dirs`: Overrides the location at which installed extensions are searched for. The default location for extensions is the value of `java.ext.dirs`.
+
+## Example
+The output from the `javap DocFooter.class` command yields the following:
+```
+Compiled from "DocFooter.java"
+public class t5750.javap.DocFooter extends java.applet.Applet {
+  java.lang.String date;
+  java.lang.String email;
+  public t5750.javap.DocFooter();
+  public void init();
+  public void paint(java.awt.Graphics);
+}
+```
+
+The output from `javap -c DocFooter.class` command yields the following:
+```
+Compiled from "DocFooter.java"
+public class t5750.javap.DocFooter extends java.applet.Applet {
+  java.lang.String date;
+
+  java.lang.String email;
+
+  public t5750.javap.DocFooter();
+    Code:
+       0: aload_0
+       1: invokespecial #1                  // Method java/applet/Applet."<init>":()V
+       4: return
+
+  public void init();
+    Code:
+       0: aload_0
+       1: sipush        500
+       4: bipush        100
+       6: invokevirtual #2                  // Method resize:(II)V
+       9: aload_0
+      10: aload_0
+      11: ldc           #3                  // String LAST_UPDATED
+      13: invokevirtual #4                  // Method getParameter:(Ljava/lang/String;)Ljava/lang/String;
+      16: putfield      #5                  // Field date:Ljava/lang/String;
+      19: aload_0
+      20: aload_0
+      21: ldc           #6                  // String EMAIL
+      23: invokevirtual #4                  // Method getParameter:(Ljava/lang/String;)Ljava/lang/String;
+      26: putfield      #7                  // Field email:Ljava/lang/String;
+      29: return
+
+  public void paint(java.awt.Graphics);
+    Code:
+       0: aload_1
+       1: new           #8                  // class java/lang/StringBuilder
+       4: dup
+       5: invokespecial #9                  // Method java/lang/StringBuilder."<init>":()V
+       8: aload_0
+       9: getfield      #5                  // Field date:Ljava/lang/String;
+      12: invokevirtual #10                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
+      15: ldc           #11                 // String  by
+      17: invokevirtual #10                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
+      20: invokevirtual #12                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
+      23: bipush        100
+      25: bipush        15
+      27: invokevirtual #13                 // Method java/awt/Graphics.drawString:(Ljava/lang/String;II)V
+      30: aload_1
+      31: aload_0
+      32: getfield      #7                  // Field email:Ljava/lang/String;
+      35: sipush        290
+      38: bipush        15
+      40: invokevirtual #13                 // Method java/awt/Graphics.drawString:(Ljava/lang/String;II)V
+      43: return
+}
+```
+
+## References
+- [javap](https://docs.oracle.com/javase/8/docs/technotes/tools/windows/javap.html)
diff --git a/doc/source/jdk8/javassistTutorial.md b/doc/source/jdk8/javassistTutorial.md
@@ -588,6 +588,43 @@ All modified class files are saved in `./dump`.
 - `IntrospectionAndCustomizationTest`
 - `BytecodeLevelApiTest`
 
+### What Is the Bytecode?
+```
+cd java-repositories/jdk8/t5750/javassist/domain
+javap -c Pair.class
+```
+```
+public class t5750.javassist.domain.Pair extends java.awt.geom.Point2D implements java.io.Serializable {
+  public t5750.javassist.domain.Pair(int, int);
+    Code:
+       0: aload_0
+       1: invokespecial #80                 // Method java/awt/geom/Point2D."<init>":()V
+       4: aload_0
+       5: iload_1
+       6: putfield      #75                 // Field x:I
+       9: aload_0
+      10: iload_2
+      11: putfield      #76                 // Field y:I
+      14: return
+
+  public void move(int, int);
+    Code:
+       0: aload_0
+       1: iload_1
+       2: putfield      #75                 // Field x:I
+       5: aload_0
+       6: iload_2
+       7: putfield      #76                 // Field y:I
+      10: return
+}
+```
+Let's analyze the bytecode instructions of the `move()` method:
+- `aload_0` instruction is loading a reference onto the stack from the local variable 0
+- `iload_1` is loading an int value from the local variable 1
+- `putfield` is setting a field `x` of our object. All operations are analogical for field `y`
+- The last instruction is a `return`
+
 ## References
 - [Getting Started with Javassist](http://www.javassist.org/tutorial/tutorial.html)
-- [Dynamic Class Loading Example](https://examples.javacodegeeks.com/core-java/dynamic-class-loading-example/)
+- [Dynamic Class Loading Example](https://examples.javacodegeeks.com/core-java/dynamic-class-loading-example/)
+- [Introduction to Javassist](https://www.baeldung.com/javassist)
diff --git a/jdk8/README.md b/jdk8/README.md
@@ -3,6 +3,7 @@
 ## Contents
 - [Lambda](../doc/source/jdk8/Lambda.md)
 - [JDK, JRE, JVM, JSE, JEE, JME](../doc/source/jdk8/jdkJre.md)
+- [javap](../doc/source/jdk8/javap.md)
 
 ### Javassist
 [Javassist Tutorial](../doc/source/jdk8/javassistTutorial.md)

diff --git a/jdk8/src/test/java/t5750/javap/DocFooter.java b/jdk8/src/test/java/t5750/javap/DocFooter.java
@@ -0,0 +1,20 @@
+package t5750.javap;
+
+import java.applet.Applet;
+import java.awt.*;
+
+public class DocFooter extends Applet {
+	String date;
+	String email;
+
+	public void init() {
+		resize(500, 100);
+		date = getParameter("LAST_UPDATED");
+		email = getParameter("EMAIL");
+	}
+
+	public void paint(Graphics g) {
+		g.drawString(date + " by ", 100, 15);
+		g.drawString(email, 290, 15);
+	}
+}