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--- java:javap [2023/04/12 14:48] – created odefta
+++ java:javap [2023/07/04 19:36] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
 ====== JDK's javap tool ======
-====== What is it ======
+===== What is it =====
-"javap" is a tool in the Java Development Kit (JDK) that displays details about the internal structure of Java classes.\\
+The javap command disassembles one or more class files. \\
+It displays details about the internal structure of Java classes.\\
 It allows us to inspect compiled Java code to better understand how Java works at the bytecode level.\\
@@ Line 12: / Line 13: @@
   * The byte code of the method, which represents the bytecode instructions used by the JVM.
   * The fields of the class, including their names and types.
+===== When to use it =====
+  * To better understand how Java code works at the bytecode level.
+  * To check method signatures and parameter data types and return value types.
+  * To check if a class contains the expected fields and methods.
+  * To check for errors or performance issues in a program.
+===== Example of usage =====
+Given the following class:
+<code java>
+package ro.medjava.generics;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+public class Erasure {
+    public static void doStuff1(List<String> ls) {
+    }
+    public static <E> void doStuff2(List<E> ls){
+    }
+    public static void main(String[] args) {
+        doStuff2(new ArrayList<>(Arrays.asList("Fred")));
+    }
+}
+</code>
+<note tip>We want to check if all the generic types are erased after the compilation.</note>
+<note important>Be sure that the javap tool is added to the PATH environment variable before using it.
+It is located in the JAVA_HOME\bin folder.
+</note>
+Command to disassemble the Erasure class is:
+<code>
+C:\JavaProject\src\main\java> javap -cp ../../../target/classes -c ro.medjava.generics.Erasure
+</code>
+<note tip>-c parameter is used to display the disassembled code.</note>
+Result:
+<code>
+public ro.medjava.generics.Erasure();
+  Code:
+: aload_0
+: invokespecial #1                // Method java/lang/Object."<init>":()V
+: return
+  public static void doStuff1(java.util.List<java.lang.String>);
+    Code:
+: return
+  public static <E> void doStuff2(java.util.List<E>);
+    Code:
+: return
+  public static void main(java.lang.String[]);
+    Code:
+: new           #2               // class java/util/ArrayList
+: dup
+: iconst_1
+: anewarray     #3               // class java/lang/String
+: dup
+: iconst_0
+: ldc           #4               // String Fred
+: aastore
+: invokestatic  #5               // Method java/util/Arrays.asList:([Ljava/lang/Object;)Ljava/util/List;
+: invokespecial #6               // Method java/util/ArrayList."<init>":(Ljava/util/Collection;)V
+: invokestatic  #7               // Method doStuff2:(Ljava/util/List;)V
+: return
+</code>
+As we can see in the parameter list of doStuff1 and doStuff2 methods, the generic types survived after compilation.\\
+Some consistency checking information still remains in the binary.\\
+But in the main method, the array list passed to the doStuff2 method does not contain any generic type after compilation.
+===== Use javap in Intellij IDEA =====
+Install the plugin "Class Decompile." (File / Settings / Plugins)
+{{:java:pasted:20230412-152124.png}}
+{{:java:pasted:20230412-152141.png}}
+Next, right click on your class and select "Show Decompile Code" option.
+Result:
+{{:java:javap_erasure.png?nolink&1536×648}}