How is String concatenation implemented in Java 9?

As written in JEP 280: Indify String Concatenation:

Change the static String-concatenation bytecode sequence generated by javac to use invokedynamic calls to JDK library functions. This will enable future optimizations of String concatenation without requiring further changes to the bytecode emmited by javac.

Here I want to understand what the use of invokedynamic calls is and how bytecode concatenation is different from invokedynamic?

The "old" way output a bunch of StringBuilder-oriented operations. Consider this program:

public class Example {
    public static void main(String[] args)
    {
        String result = args[0] + "-" + args[1] + "-" + args[2];
        System.out.println(result);
    }
}

If we compile that with JDK 8 or earlier and then use javap -c Example to see the bytecode, we see something like this:

public class Example {
  public Example();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: new           #2                  // class java/lang/StringBuilder
       3: dup
       4: invokespecial #3                  // Method java/lang/StringBuilder."<init>":()V
       7: aload_0
       8: iconst_0
       9: aaload
      10: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      13: ldc           #5                  // String -
      15: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      18: aload_0
      19: iconst_1
      20: aaload
      21: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      24: ldc           #5                  // String -
      26: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      29: aload_0
      30: iconst_2
      31: aaload
      32: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      35: invokevirtual #6                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      38: astore_1
      39: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;
      42: aload_1
      43: invokevirtual #8                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      46: return
}

As you can see, it creates a StringBuilder and uses append. This is famous fairly inefficient as the default capacity of the built-in buffer in StringBuilder is only 16 chars, and there's no way for the compiler to know to allocate more in advance, so it ends up having to reallocate. It's also a bunch of method calls. (Note that the JVM can sometimes detect and rewrite these patterns of calls to make them more efficient, though.)

Let's look at what Java 9 generates:

public class Example {
  public Example();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: aload_0
       1: iconst_0
       2: aaload
       3: aload_0
       4: iconst_1
       5: aaload
       6: aload_0
       7: iconst_2
       8: aaload
       9: invokedynamic #2,  0              // InvokeDynamic #0:makeConcatWithConstants:(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
      14: astore_1
      15: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
      18: aload_1
      19: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      22: return
}

Oh my but that's shorter. :-) It makes a single call to makeConcatWithConstants from StringConcatFactory, which says this in its Javadoc:

Methods to facilitate the creation of String concatenation methods, that can be used to efficiently concatenate a known number of arguments of known types, possibly after type adaptation and partial evaluation of arguments. These methods are typically used as bootstrap methods for invokedynamic call sites, to support the string concatenation feature of the Java Programming Language.

Before going into the details of the invokedynamic implementation used for optimisation of String concatenation, in my opinion, one must get some background over What's invokedynamic and how do I use it?

The invokedynamic instruction simplifies and potentially improves implementations of compilers and runtime systems for dynamic languages on the JVM. It does this by allowing the language implementer to define custom linkage behavior with the invokedynamic instruction which involves the following the below steps.

I would probably try and take you through these with the changes that were brought along for the implementation of String concatenation optimisation.

Defining the Bootstrap Method:- With Java9, the bootstrap methods for invokedynamic call sites, to support the string concatenation primarily makeConcat and makeConcatWithConstants were introduced with the StringConcatFactory implementation.

The use of invokedynamic provides an alternative to select a translation strategy until runtime. The translation strategy used in StringConcatFactory is similar to the LambdaMetafactory as introduced in the previous java version. Additionally one of the goals of the JEP mentioned in the question is to stretch these strategies further.
Specifying Constant Pool Entries:- These are the additional static arguments to the invokedynamic instruction other than (1) MethodHandles.Lookup object which is a factory for creating method handles in the context of the invokedynamic instruction,(2) a String object, the method name mentioned in the dynamic call site and (3) the MethodType object, the resolved type signature of the dynamic call site.

There are already linked during the linkage of the code. At runtime, the bootstrap method runs and links in the actual code doing the concatenation. It rewrites the invokedynamic call with an appropriate invokestatic call. This loads the constant string from the constant pool, the bootstrap method static args are leveraged to pass these and other constants straight to the bootstrap method call.
Using the invokedynamic Instruction:- This offers the facilities for a lazy linkage, by providing the means to bootstrap the call target once, during the initial invocation. The concrete idea for optimisation here is to replace the entire StringBuilder.append dance with a simple invokedynamic call to java.lang.invoke.StringConcatFactory, that will accept the values in the need of concatenation.

The Indify String Concatenation proposal states with an example the benchmarking of the application with Java9 where a similar method as shared by @T.J. Crowder is compiled and the difference in the bytecode is fairly visible between the varying implementation.

I'll slightly add a bit of details here. The main part to get is that how string concatenation is done is a runtime decision, not a compile time one anymore. Thus it can change, meaning that you have compiled your code once against java-9 and it can change the underlying implementation however it pleases, without the need to re-compile.

And the second point is that at the moment there are 6 possible strategies for concatenation of String:

 private enum Strategy {
    /**
     * Bytecode generator, calling into {@link java.lang.StringBuilder}.
     */
    BC_SB,

    /**
     * Bytecode generator, calling into {@link java.lang.StringBuilder};
     * but trying to estimate the required storage.
     */
    BC_SB_SIZED,

    /**
     * Bytecode generator, calling into {@link java.lang.StringBuilder};
     * but computing the required storage exactly.
     */
    BC_SB_SIZED_EXACT,

    /**
     * MethodHandle-based generator, that in the end calls into {@link java.lang.StringBuilder}.
     * This strategy also tries to estimate the required storage.
     */
    MH_SB_SIZED,

    /**
     * MethodHandle-based generator, that in the end calls into {@link java.lang.StringBuilder}.
     * This strategy also estimate the required storage exactly.
     */
    MH_SB_SIZED_EXACT,

    /**
     * MethodHandle-based generator, that constructs its own byte[] array from
     * the arguments. It computes the required storage exactly.
     */
    MH_INLINE_SIZED_EXACT
}

You can choose any of them via a parameter : -Djava.lang.invoke.stringConcat. Notice that StringBuilder is still an option.

How is String concatenation implemented in Java 9?

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