Lambdas in Java Bytecode - Java Closures and Lambda - page 144

Listing 8-3. Java Code for a Bound Instance Method Reference with Its Bytecode (Abbreviated)
1 // Listing3.java, which is compiled to Listing3.class
2 import java.util.function.*;
3 public class Listing3 {
4 public Consumer<String> getConsumer() {
5 return System.out::println;
6 }
7 }
8
9 // Abbreviated result of executing /usr/bin/javap -v -l -p -s -c Listing3 in the same
directory as Listing3.class
10 public class Listing3
11 BootstrapMethods:
12 0: #25 invokestatic java/lang/invoke/LambdaMetafactory.metafactory:(
Ljava/lang/invoke/MethodHandles$Lookup;
Ljava/lang/String;
Ljava/lang/invoke/MethodType;
Ljava/lang/invoke/MethodType;
Ljava/lang/invoke/MethodHandle;
Ljava/lang/invoke/MethodType;
)Ljava/lang/invoke/CallSite;
13 Method arguments:
14 #26 (Ljava/lang/Object;)V
15 #27 invokevirtual java/io/PrintStream.println:(Ljava/lang/String;)V
16 #28 (Ljava/lang/String;)V
17 {
18 public java.util.function.Consumer<java.lang.String> getConsumer();
19 LocalVariableTable:
20 Start Length Slot Name Signature
21 0 14 0 this LListing3;
22 Code:
23 stack=2, locals=1, args_size=1
24 0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
25 3: dup
26 4: invokevirtual #3
// Method java/lang/Object.getClass:()Ljava/lang/Class;
27 7: pop
28 8: invokedynamic #4, 0 // InvokeDynamic
// #0:accept:(Ljava/io/PrintStream;)Ljava/util/
function/Consumer;
29 13: areturn
30 }
If we compare this to the function in 8-2, we see that there is a bit more going on. The first thing is
the getstatic call, which loads System.out onto the stack. Eventually, we will pass that instance into
our invokedynamic call on instruction 8, which will again go by way of the bootstrap method to produce
our Consumer. The bootstrap method is practically identical to what we saw with the static method
reference, except that our method is being invoked using invokevirtual instead of invokestatic . Whereas
invokestatic makes the static method call, invokevirtual makes an instance method call. So that is our
method reference, and the instance to invoke it on is also passed into the bootstrap method. Aside from that
difference, this is all very similar to what we saw before. But what is that stuff in instructions 3, 4, and 7?