Recently I was presented with the following error when serializing a lambda with Kryo:
com.esotericsoftware.kryo.KryoException: java.lang.IllegalArgumentException: Unable to serialize Java Lambda expression, unless explicitly declared e.g., Runnable r = (Runnable & Serializable) () -> System.out.println("Hello world!"); If you do not recognise the (Runnable & Serializable) syntax, don't worry, it is merely stating that the lambda must implement two types. This is called Type Intersection. Personally, I have never needed to use this myself, so have never really thought about it. Serializable is a bit of a unique interface in this regards, as there is nothing you actually need to implement.
Without making this cast, the lambda will be considered unserializable, which does not make Kryo happy.
As someone who doesn't look at bytecode very often, I find it amazing how big the difference is when adding and extra casting of & Serializable. The examples below demonstrate this. For clarity, I used the following command to generate bytecode from the code snippets:
javap -c -p target.classes.dev.lankydan.IntersectionCasting Before doing any casting:
public class IntersectionCasting { public static void main(String[] args) { Function<String, String> function = (message) -> "Kryo please serialize this message '" + message + "'"; } } The generated bytecode is:
public class dev.lankydan.IntersectionCasting { public dev.lankydan.IntersectionCasting(); Code: 0: aload_0 1: invokespecial #1 // Method java/lang/Object."<init>":()V 4: return public static void main(java.lang.String[]); Code: 0: invokedynamic #2, 0 // InvokeDynamic #0:apply:()Ljava/util/function/Function; 5: astore_1 6: return private static java.lang.String lambda$main$0(java.lang.String); Code: 0: new #3 // class java/lang/StringBuilder 3: dup 4: invokespecial #4 // Method java/lang/StringBuilder."<init>":()V 7: ldc #5 // String Kryo please serialize this message ' 9: invokevirtual #6 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 12: aload_0 13: invokevirtual #6 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 16: ldc #7 // String ' 18: invokevirtual #6 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 21: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String; 24: areturn } After casting:
public class IntersectionCasting { public static void main(String[] args) { Function<String, String> function = (Function<String, String> & Serializable) (message) -> "Kryo please serialize this message '" + message + "'"; } } The bytecode becomes:
public class dev.lankydan.IntersectionCasting { public dev.lankydan.IntersectionCasting(); Code: 0: aload_0 1: invokespecial #1 // Method java/lang/Object."<init>":()V 4: return public static void main(java.lang.String[]); Code: 0: invokedynamic #2, 0 // InvokeDynamic #0:apply:()Ljava/util/function/Function; 5: checkcast #3 // class java/io/Serializable 8: checkcast #4 // class java/util/function/Function 11: astore_1 12: return private static java.lang.Object $deserializeLambda$(java.lang.invoke.SerializedLambda); Code: 0: aload_0 1: invokevirtual #5 // Method java/lang/invoke/SerializedLambda.getImplMethodName:()Ljava/lang/String; 4: astore_1 5: iconst_m1 6: istore_2 7: aload_1 8: invokevirtual #6 // Method java/lang/String.hashCode:()I 11: lookupswitch { // 1 -1657128837: 28 default: 39 } 28: aload_1 29: ldc #7 // String lambda$main$2cf54983$1 31: invokevirtual #8 // Method java/lang/String.equals:(Ljava/lang/Object;)Z 34: ifeq 39 37: iconst_0 38: istore_2 39: iload_2 40: lookupswitch { // 1 0: 60 default: 135 } 60: aload_0 61: invokevirtual #9 // Method java/lang/invoke/SerializedLambda.getImplMethodKind:()I 64: bipush 6 66: if_icmpne 135 69: aload_0 70: invokevirtual #10 // Method java/lang/invoke/SerializedLambda.getFunctionalInterfaceClass:()Ljava/lang/String; 73: ldc #11 // String java/util/function/Function 75: invokevirtual #12 // Method java/lang/Object.equals:(Ljava/lang/Object;)Z 78: ifeq 135 81: aload_0 82: invokevirtual #13 // Method java/lang/invoke/SerializedLambda.getFunctionalInterfaceMethodName:()Ljava/lang/String; 85: ldc #14 // String apply 87: invokevirtual #12 // Method java/lang/Object.equals:(Ljava/lang/Object;)Z 90: ifeq 135 93: aload_0 94: invokevirtual #15 // Method java/lang/invoke/SerializedLambda.getFunctionalInterfaceMethodSignature:()Ljava/lang/String; 97: ldc #16 // String (Ljava/lang/Object;)Ljava/lang/Object; 99: invokevirtual #12 // Method java/lang/Object.equals:(Ljava/lang/Object;)Z 102: ifeq 135 105: aload_0 106: invokevirtual #17 // Method java/lang/invoke/SerializedLambda.getImplClass:()Ljava/lang/String; 109: ldc #18 // String dev/lankydan/IntersectionCasting 111: invokevirtual #12 // Method java/lang/Object.equals:(Ljava/lang/Object;)Z 114: ifeq 135 117: aload_0 118: invokevirtual #19 // Method java/lang/invoke/SerializedLambda.getImplMethodSignature:()Ljava/lang/String; 121: ldc #20 // String (Ljava/lang/String;)Ljava/lang/String; 123: invokevirtual #12 // Method java/lang/Object.equals:(Ljava/lang/Object;)Z 126: ifeq 135 129: invokedynamic #2, 0 // InvokeDynamic #0:apply:()Ljava/util/function/Function; 134: areturn 135: new #21 // class java/lang/IllegalArgumentException 138: dup 139: ldc #22 // String Invalid lambda deserialization 141: invokespecial #23 // Method java/lang/IllegalArgumentException."<init>":(Ljava/lang/String;)V 144: athrow private static java.lang.String lambda$main$2cf54983$1(java.lang.String); Code: 0: new #24 // class java/lang/StringBuilder 3: dup 4: invokespecial #25 // Method java/lang/StringBuilder."<init>":()V 7: ldc #26 // String Kryo please serialize this message ' 9: invokevirtual #27 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 12: aload_0 13: invokevirtual #27 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 16: ldc #28 // String ' 18: invokevirtual #27 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 21: invokevirtual #29 // Method java/lang/StringBuilder.toString:()Ljava/lang/String; 24: areturn } Now, I don't really know how to read bytecode, but even I can see that there is a lot more going on in the version with the & Serializable cast.
If you can explain what is going on there, then be my guest and give me a shout.
One thing I can read from the bytecode above, is the references to SerializedLambda which were not there before. This class is used by compilers and libraries to ensure that lambdas deserialise correctly. Making the intersection cast of Function<String, String> & Serializable changes the underlying type of the lambda, allowing a library like Kryo to properly understand how to deserialise lambdas given to it.
Adding this extra casting of & Serializable is one possible solution to allow Kryo to deserialise lambdas. An alternative route involves creating a new interface that extends both the underlying Function type that you need, along with Serializable. This is useful when putting together a library or API for others to consume. Allowing them to focus purely on implementing their code, rather than worrying about providing the correct casting to satisfy the serialisation of their lambdas.
You could use an interface like the one below:
interface SerializableLambda extends Function<String, String>, Serializable {} This can then be used to replace the casting in the previous example:
public class IntersectionCasting { public static void main(String[] args) { SerializableLambda function = (message) -> "Kryo please serialize this message '" + message + "'"; } interface SerializableLambda extends Function<String, String>, Serializable {} } I have added the bytecode generated by this change below:
public class dev.lankydan.IntersectionCasting { public dev.lankydan.IntersectionCasting(); Code: 0: aload_0 1: invokespecial #1 // Method java/lang/Object."<init>":()V 4: return public static void main(java.lang.String[]); Code: // NO CASTING // Mention of casting is removed and reference to new interface is added 0: invokedynamic #2, 0 // InvokeDynamic #0:apply:()Ldev/lankydan/IntersectionCasting$SerializableLambda; 5: astore_1 6: return private static java.lang.Object $deserializeLambda$(java.lang.invoke.SerializedLambda); Code: 0: aload_0 1: invokevirtual #3 // Method java/lang/invoke/SerializedLambda.getImplMethodName:()Ljava/lang/String; 4: astore_1 5: iconst_m1 6: istore_2 7: aload_1 8: invokevirtual #4 // Method java/lang/String.hashCode:()I 11: lookupswitch { // 1 -1657128837: 28 default: 39 } 28: aload_1 29: ldc #5 // String lambda$main$2cf54983$1 31: invokevirtual #6 // Method java/lang/String.equals:(Ljava/lang/Object;)Z 34: ifeq 39 37: iconst_0 38: istore_2 39: iload_2 40: lookupswitch { // 1 0: 60 default: 135 } 60: aload_0 61: invokevirtual #7 // Method java/lang/invoke/SerializedLambda.getImplMethodKind:()I 64: bipush 6 66: if_icmpne 135 69: aload_0 70: invokevirtual #8 // Method java/lang/invoke/SerializedLambda.getFunctionalInterfaceClass:()Ljava/lang/String; 73: ldc #9 // String dev/lankydan/IntersectionCasting$SerializableLambda 75: invokevirtual #10 // Method java/lang/Object.equals:(Ljava/lang/Object;)Z 78: ifeq 135 81: aload_0 82: invokevirtual #11 // Method java/lang/invoke/SerializedLambda.getFunctionalInterfaceMethodName:()Ljava/lang/String; 85: ldc #12 // String apply 87: invokevirtual #10 // Method java/lang/Object.equals:(Ljava/lang/Object;)Z 90: ifeq 135 93: aload_0 94: invokevirtual #13 // Method java/lang/invoke/SerializedLambda.getFunctionalInterfaceMethodSignature:()Ljava/lang/String; 97: ldc #14 // String (Ljava/lang/Object;)Ljava/lang/Object; 99: invokevirtual #10 // Method java/lang/Object.equals:(Ljava/lang/Object;)Z 102: ifeq 135 105: aload_0 106: invokevirtual #15 // Method java/lang/invoke/SerializedLambda.getImplClass:()Ljava/lang/String; 109: ldc #16 // String dev/lankydan/IntersectionCasting 111: invokevirtual #10 // Method java/lang/Object.equals:(Ljava/lang/Object;)Z 114: ifeq 135 117: aload_0 118: invokevirtual #17 // Method java/lang/invoke/SerializedLambda.getImplMethodSignature:()Ljava/lang/String; 121: ldc #18 // String (Ljava/lang/String;)Ljava/lang/String; 123: invokevirtual #10 // Method java/lang/Object.equals:(Ljava/lang/Object;)Z 126: ifeq 135 129: invokedynamic #2, 0 // InvokeDynamic #0:apply:()Ldev/lankydan/IntersectionCasting$SerializableLambda; 134: areturn 135: new #19 // class java/lang/IllegalArgumentException 138: dup 139: ldc #20 // String Invalid lambda deserialization 141: invokespecial #21 // Method java/lang/IllegalArgumentException."<init>":(Ljava/lang/String;)V 144: athrow private static java.lang.String lambda$main$2cf54983$1(java.lang.String); Code: 0: new #22 // class java/lang/StringBuilder 3: dup 4: invokespecial #23 // Method java/lang/StringBuilder."<init>":()V 7: ldc #24 // String Kryo please serialize this message ' 9: invokevirtual #25 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 12: aload_0 13: invokevirtual #25 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 16: ldc #26 // String ' 18: invokevirtual #25 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 21: invokevirtual #27 // Method java/lang/StringBuilder.toString:()Ljava/lang/String; 24: areturn } Most of it is the same, with some new references to the SerializableLambda interface and the removal of the original intersection cast.
As mentioned before, this solution is ideal for library and API authors as it allows developers to write code as usual without having to worry about casting (for example, if the library uses Kryo under the hood). Furthermore, since the interface extends Function which is a @FunctionalInterface, developers can nicely write lambdas/functions and don't even have to mention the interface if passing it directly into another function or constructor. I personally went down this route when designing a new API for Corda. I wanted to provide the most accessible API for developers to use, while still providing an API that works (I can't let Kryo blow up...).
In conclusion, in this post which lacks a lot of information and is littered with extended snippets of bytecode, you need to take away two things. You can make a Java lambda/function serializable through type intersection, and you can ensure that your own APIs are clean by creating a new interface that extends both your desired function type and Serializable. These are both routes that should be considered when using a serialisation library like Kryo.
If you enjoyed this post or found it helpful (or both) then please feel free to follow me on Twitter at @LankyDanDev and remember to share with anyone else who might find this useful!
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