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Using static variables in local scope in Java.
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| import java.util.regex.Pattern; | |
| public class Test { | |
| public static void main(String[] args) { | |
| check("a"); | |
| check("b"); | |
| } | |
| static Pattern compile(String pattern) { | |
| System.out.println("compile(" + pattern + ")"); | |
| return Pattern.compile(pattern); | |
| } | |
| static void check(String string) { | |
| // Compiling the pattern only once: Good | |
| // Keeping the pattern local to the method: Good | |
| // Capturing scope: Egh... | |
| var patterns = new Object() { | |
| static final Pattern P_CHECK = compile("a"); | |
| }; | |
| System.out.println("check(" + string + "): " | |
| + patterns.P_CHECK.matcher(string).find()); | |
| } | |
| } |
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| import java.util.regex.Pattern; | |
| public class Test { | |
| public static void main(String[] args) { | |
| check("a"); | |
| check("b"); | |
| } | |
| static Pattern compile(String pattern) { | |
| System.out.println("compile(" + pattern + ")"); | |
| return Pattern.compile(pattern); | |
| } | |
| static void check(String string) { | |
| // Compiling the pattern only once: Good | |
| // Keeping the pattern local to the method: Good | |
| // Does not capture the scope: Good | |
| class Patterns { | |
| static final Pattern P_CHECK = compile("a"); | |
| } | |
| System.out.println("check(" + string + "): " | |
| + Patterns.P_CHECK.matcher(string).find()); | |
| } | |
| } |
Bytcode of the first version:
▶ javap -c Test\$1
Compiled from "Test.java"
class Test$1 {
static final java.util.regex.Pattern P_CHECK;
Test$1();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
static {};
Code:
0: ldc #7 // String a
2: invokestatic #9 // Method Test.compile:(Ljava/lang/String;)Ljava/util/regex/Pattern;
5: putstatic #15 // Field P_CHECK:Ljava/util/regex/Pattern;
8: return
}
▶ javap -c Test
Compiled from "Test.java"
public class Test {
public Test();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: ldc #7 // String a
2: invokestatic #9 // Method check:(Ljava/lang/String;)V
5: ldc #15 // String b
7: invokestatic #9 // Method check:(Ljava/lang/String;)V
10: return
static java.util.regex.Pattern compile(java.lang.String);
Code:
0: getstatic #17 // Field java/lang/System.out:Ljava/io/PrintStream;
3: aload_0
4: invokedynamic #23, 0 // InvokeDynamic #0:makeConcatWithConstants:(Ljava/lang/String;)Ljava/lang/String;
9: invokevirtual #27 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
12: aload_0
13: invokestatic #32 // Method java/util/regex/Pattern.compile:(Ljava/lang/String;)Ljava/util/regex/Pattern;
16: areturn
static void check(java.lang.String);
Code:
0: new #38 // class Test$1
3: dup
4: invokespecial #40 // Method Test$1."<init>":()V
7: astore_1
8: getstatic #17 // Field java/lang/System.out:Ljava/io/PrintStream;
11: aload_0
12: aload_1
13: pop
14: getstatic #41 // Field Test$1.P_CHECK:Ljava/util/regex/Pattern;
17: aload_0
18: invokevirtual #45 // Method java/util/regex/Pattern.matcher:(Ljava/lang/CharSequence;)Ljava/util/regex/Matcher;
21: invokevirtual #49 // Method java/util/regex/Matcher.find:()Z
24: invokedynamic #55, 0 // InvokeDynamic #1:makeConcatWithConstants:(Ljava/lang/String;Z)Ljava/lang/String;
29: invokevirtual #27 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
32: return
}
The generated class Test$1 is identical to the Test$Patterns class, but the check implementation is unnecessarily creating a new instance of Test$1, resulting in 8 extra, unnecessary bytecode instructions.
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Bytecode of the second version: