Java string concatenation '+' behavior: compiler-dependent optimization limits

Answer

Both javac and Eclipse ECJ generally emit a StringBuilder chain for any concatenation that involves more than two operands or is not a simple binary expression. For a two‑operand expression where the left side is a string literal and the right side is a single String value, they may replace the StringBuilder with a direct call to String.concat to avoid creating a temporary builder.

Explanation

Heuristics in OpenJDK javac

• Binary literal + variable: uses String.concat if the right operand is a String or a known constant.
• Anything else (more than two operands, method calls, numeric types, etc.): emits a StringBuilder chain.
• From Java 9+, the compiler can emit invokedynamic via StringConcatFactory when the –XDstringConcat=enabled flag is set.

Heuristics in Eclipse ECJ

• Similar binary literal + variable rule, but the threshold is lower: it keeps using String.concat for two operands even when the right side is a non‑constant string expression.
• Three or more operands always fall back to StringBuilder.
• ECJ also supports the –XDstringConcat=enabled flag for invokedynamic in recent releases.

Bytecode Patterns to Look For

• StringBuilder chain: new StringBuilder, dup, invokespecial <init>, several invokevirtual append calls, followed by invokevirtual toString.
• Direct concat: aload_#, invokevirtual java/lang/String.concat.
• Invokedynamic (Java 9+): invokedynamic instruction referencing StringConcatFactory.

Verification Steps

1. Create a minimal test class:

   public class Test {
       static String two(String a) { return "a" + a; }
       static String three(String a, String b) { return a + b + "c"; }
   }

2. Compile with the compiler under investigation:

   javac Test.java          # OpenJDK
   ecj Test.java             # Eclipse ECJ
   

3. Disassemble the bytecode:

   javap -c Test
   

4. Inspect the output for the patterns described above.

Optional: Enable Invokedynamic

To see the invokedynamic path, compile with

javac -XDstringConcat=enabled Test.java

or for ECJ, use the equivalent flag if available.

Missing Diagnostic Detail

To tailor the recommendation precisely, please specify the exact compiler version and any flags you use (e.g., –XDstringConcat=enabled). The heuristics can shift between releases.