Kotlin is a great JVM language packed with tons of features that make our jobs easier and more enjoyable. It has features that I would have loved to have seen in Java such as properties, top level functions, properties, companion objects, single line DTO classes and so on.
In this series of posts we’re going to look under the hood to see what those features of Kotlin are made of. My method will be to write some simple code in Kotlin and de-compile the generated class files and hopefully learn some things. I’ll decompile using a variety of methods in hopes to learn as much as I can. This is a learning process for me as I’ve never decompiled Java code.
For this post, I looked at the compiled code using Bytecode Viewer.
Classes
To start this series, we’ll take a look at a couple of very simple classes and some ways they differ from Java.
SimpleClass.kt
1 package io.github.johnbrainard.exposed
2
3 class SimpleClass(var someData:String, someParam:String) {
4 fun simpleMethod() {
5 println("Hello, World!")
6 }
7 } 1 /*
2 * Decompiled with CFR 0_102.
3 *
4 * Could not load the following classes:
5 * io.github.johnbrainard.exposed.SimpleClass
6 * jet.runtime.typeinfo.JetValueParameter
7 * kotlin.io.IoPackage
8 * kotlin.jvm.internal.Intrinsics
9 * kotlin.jvm.internal.KotlinClass
10 * kotlin.jvm.internal.KotlinClass$Kind
11 * kotlin.jvm.internal.Reflection
12 * kotlin.reflect.KClass
13 * org.jetbrains.annotations.NotNull
14 */
15 package io.github.johnbrainard.exposed;
16
17 import jet.runtime.typeinfo.JetValueParameter;
18 import kotlin.io.IoPackage;
19 import kotlin.jvm.internal.Intrinsics;
20 import kotlin.jvm.internal.KotlinClass;
21 import kotlin.jvm.internal.Reflection;
22 import kotlin.reflect.KClass;
23 import org.jetbrains.annotations.NotNull;
24
25 @KotlinClass(abiVersion=23, kind=KotlinClass.Kind.CLASS, data={"\u001d\u0004)Y1+[7qY\u0016\u001cE.Y:t\u0015\tIwN\u0003\u0004hSRDWO\u0019\u0006\rU>DgN\u0019:bS:\f'\u000f\u001a\u0006\bKb\u0004xn]3e\u0015\r\te.\u001f\u0006\u0007W>$H.\u001b8\u000b\rqJg.\u001b;?\u0015!\u0019x.\\3ECR\f'BB*ue&twMC\u0005t_6,\u0007+\u0019:b[*!!.\u0019<b\u0015\u0011a\u0017M\\4\u000b\u0017\u001d,GoU8nK\u0012\u000bG/\u0019\u0006\fg\u0016$8k\\7f\t\u0006$\u0018M\u0003\u0007tS6\u0004H.Z'fi\"|GM\u0003\u0003V]&$(J\u0003\u0002\u0011\u0003)!\u0001\u0002\u0001\t\u0003\u0015\u0011A\u0011\u0001E\u0002\u000b\t!\u0011\u0001\u0003\u0002\u0006\u0007\u0011\r\u0001\u0002\u0001\u0007\u0001\u000b\u0005A1!B\u0002\u0005\u0006!\u0015A\u0002A\u0003\u0004\t\u000bAI\u0001\u0004\u0001\u0006\u0003!-QA\u0001\u0003\u0005\u0011\u0019)!\u0001\"\u0003\t\n\u0015\u0019AQ\u0001\u0005\t\u0019\u0001!\u0001\u0001\u0004\u0002\u001a\u0005\u0015\t\u0001bA\u0017\u0016\t\u0001g\u0001\u0004B\u0011\u0003\u000b\u0005A9!V\u0002\u000f\u000b\r!A!C\u0001\t\u000b5\u0019AQB\u0005\u0002\u0011\u0015\tR\u0001B\u0004\n\u0003\u0011\u0001Q\"\u0001\u0005\u0006['!\u0001\u0001g\u0004\"\u0005\u0015\t\u00012B)\u0004\u0007\u0011=\u0011\"\u0001\u0003\u0001ky)Q\u0004Br\u00011\u000fij\u0001\u0002\u0001\t\t5\u0011Q!\u0001E\u0004!\u000e\u0001QT\u0002\u0003\u0001\u0011\u0015i!!B\u0001\t\bA\u001b\t!\t\u0002\u0006\u0003!\u0011\u0011kA\u0004\u0005\b%\tA\u0001A\u0007\u0002\u0011\u0015i\u0011\u0001C\u0003"})
26 public final class SimpleClass {
27 public static final /* synthetic */ KClass $kotlinClass;
28 @NotNull
29 private String someData;
30
31 static {
32 $kotlinClass = Reflection.createKotlinClass((Class)SimpleClass.class);
33 }
34
35 public final void simpleMethod() {
36 IoPackage.println((Object)"Hello, World!");
37 }
38
39 @NotNull
40 public final String getSomeData() {
41 return this.someData;
42 }
43
44 public final void setSomeData(@JetValueParameter(name="<set-?>") @NotNull String string) {
45 Intrinsics.checkParameterIsNotNull((Object)string, (String)"<set-?>");
46 this.someData = string;
47 }
48
49 public SimpleClass(@JetValueParameter(name="someData") @NotNull String someData, @JetValueParameter(name="someParam") @NotNull String someParam) {
50 Intrinsics.checkParameterIsNotNull((Object)someData, (String)"someData");
51 Intrinsics.checkParameterIsNotNull((Object)someParam, (String)"someParam");
52 this.someData = someData;
53 }
54 }One thing to note here is that Kotlin removes a lot of the boilerplate code that you’re required to write in Java. Our 7 lines of Kotlin expands to 40 lines of Java!
The decompiled code is mostly a pretty standard Java class. The var someData:String parameter to the SimpleClass constructor is expanded to getter and setter methods. Class propertiees can be declared and initialized in the constructor saving several lines of code if all you need to write is a simple DTO type class.
Another thing to notice is that Kotlin classes are final by default. If you want to be able to extend your classes, you’ll have to include the open attribute.
Kotlin creates a static KClass field which stores a reference to KClassImpl if you have kotlin-reflect.jar in your classpath. Otherwise, it’s an instance of ClassReference. A quick scan of the source code for KClassImpl reveals that the KClass property on Kotlin classes provides convenience methods over the Java reflection api.
The someData constructor parameter was not declared as nullable, so Kotlin generated null checks in the constructor as well as the setter method for that property.
SimpleDataClass.kt
Simply adding the data attribute to your class adds a lot of extra functionality for free that would require several lines of boilerplate code in Java. The decompiled java class is 81 lines compared to the 7 lines of Kotlin!
Jetbrains is adding restrictions to data classes in M13 that are detailed in this blog post. Essentially, data classes will be final and only able to implement interfaces and not extend other classes.
1 package io.github.johnbrainard.exposed
2
3 data class SimpleDataClass(var someData:String, someParam:String) {
4 fun simpleMethod() {
5 println("Hello, ${someData}")
6 }
7 }This is the extra functionality that the data attribute gives you in the current milestone (M12)
1 public final class SimpleDataClass {
2
3 // ... removed code we've already seen from SimpleClass.kt
4
5 @NotNull
6 public final String component1() {
7 return this.someData;
8 }
9
10 @NotNull
11 public final SimpleDataClass copy(@JetValueParameter(name="someData") @NotNull String someData, @JetValueParameter(name="someParam") @NotNull String someParam) {
12 Intrinsics.checkParameterIsNotNull((Object)someData, (String)"someData");
13 Intrinsics.checkParameterIsNotNull((Object)someParam, (String)"someParam");
14 return new SimpleDataClass(someData, someParam);
15 }
16
17 @NotNull
18 public static /* synthetic */ SimpleDataClass copy$default(SimpleDataClass simpleDataClass, String string, String string2, int n) {
19 if ((n & 1) == 0) return simpleDataClass.copy(string, string2);
20 string = simpleDataClass.someData;
21 return simpleDataClass.copy(string, string2);
22 }
23
24 public String toString() {
25 return "SimpleDataClass(someData=" + this.someData + ")";
26 }
27
28 public int hashCode() {
29 String string = this.someData;
30 if (string == null) return 0;
31 int n = string.hashCode();
32 return n;
33 }
34
35 public boolean equals(Object object) {
36 if (this == object) return true;
37 if (!(object instanceof SimpleDataClass)) return false;
38 SimpleDataClass simpleDataClass = (SimpleDataClass)object;
39 if (!Intrinsics.areEqual((Object)this.someData, (Object)simpleDataClass.someData)) return false;
40 return true;
41 }
42 }Kotlin allows you to do Multi-Declarations and does this by using the componentN methods. In SimpleDataClass, the componentN method is only generated for the someData property. If someParam were declared with var or val, component2 would have been generated.
Kotlin allows you to create copies of your data classes with the option to change the value of one or more properties. It does this by using the generated copy methods. The first method constructs an entirely new object if you pass in values for both parameters. The second copy method, copy$default is generated to handle default parameters. For instance, if we invoke copy like the following:
val copy1 = instance1.copy(someParam="foo")The generated java code will be:
SimpleDataClass copy1 = SimpleDataClass.copy$default((SimpleDataClass)instance1, (String)null, (String)"foo", (int)1);We’ll look at how default parameters are dealt with in Kotlin in more detail in a future post.
The generated toString, hashCode and equals methods are pretty standard and don’t really require any analyzing. It’s nice to know that we can get this functionality for free in Kotlin and be able to spend more of our time writing business logic.
And so…
This simple example demonstrates just the beginning of what Kotlin buys you in terms of productivity. It can save you dozens of lines of code in just simple classes or hundreds or thousands in larger projects, as well as spare you the headaches of dealing with bugs related to typographical errors. As Kotlin evolves and approaches 1.0, some of these things will change. You can already see some differences if you browse through the Kotlin source code on Github.
If you found this exercise to be useful, please leave a comment. I’d love to hear your thoughts and suggestions for future blog posts.