JVM编译期字符串联接优化分析

JVM编译期字符串连接优化分析

为了研究javac对于String相关代码的字节码优化，我做了如下测试。

    测试环境：

       $ javac -version

       javac 1.6.0_23

       $ java -version

       java version "1.6.0_23"

       OpenJDK Runtime Environment (IcedTea6 1.11pre) (6b23~pre11-0ubuntu1.11.10.2)

       OpenJDK 64-Bit Server VM (build 20.0-b11, mixed mode)

    1.编写代码：

String name = "ab"+"cd";
String name1 = "ab" + new String("cd");
String name2 = "ab"+1+"cd";
String name3 = "ab"+"c" + new String("d");
StringBuffer name4 = new StringBuffer("e");
name4.append("f").append("g");

    2.执行编译： javac StringAdd.java

    生成字节码StringAdd.class

    3.反编译：javap -c -l -verbose StringAdd > StringAdd.javap

    生成反编译文件。

    main方法如下（此处只列出主要部分，完整文件见附件）：

   0:    ldc #2; //String abcd

#字符串常量连接，已经过常量折叠。

   2:    astore_1

   3:    new #3; //class java/lang/StringBuilder

   6:    dup

   7:    invokespecial #4; //Method java/lang/StringBuilder."<init>":()V

   10:   ldc #5; //String ab

   12:   invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

   15:   new #7; //class java/lang/String

   18:   dup

   19:   ldc #8; //String cd 。

   21:   invokespecial #9; //Method java/lang/String."<init>":(Ljava/lang/String;)V

   24:   invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

   27:   invokevirtual #10; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;

   30:   astore_2

# 通过构造方法建立的String常量没有折叠，但字符串连接操作已被StringBuilder取代

   31:   ldc #11; //String ab1cd

   33:   astore_3

# 有类型转换的常量字符串连接操作也被已常量折叠方式优化

   34:   new #3; //class java/lang/StringBuilder

   37:   dup

   38:   invokespecial #4; //Method java/lang/StringBuilder."<init>":()V

   41:   ldc #12; //String abc

   43:   invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

   46:   new #7; //class java/lang/String

   49:   dup

   50:   ldc #13; //String d

   52:   invokespecial #9; //Method java/lang/String."<init>":(Ljava/lang/String;)V

   55:   invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

   58:   invokevirtual #10; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;

   61:   astore 4

# 同时包含字面常量和通过构造方法构造的String的连接，字面常量部分被已常量折叠方式优化，剩余部分的字符串连接被StringBuilder取代。

   63:   new #14; //class java/lang/StringBuffer

   66:   dup

   67:   ldc #15; //String e

   69:   invokespecial #16; //Method java/lang/StringBuffer."<init>":(Ljava/lang/String;)V

   72:   astore 5

   74:   aload 5

   76:   ldc #17; //String f

   78:   invokevirtual #18; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;

   81:   ldc #19; //String g

   83:   invokevirtual #18; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;

# 通过StringBuffer构造字符串没有被优化。

   86:   pop

   87:   return

    分析：

    通过上面对反编译字节码的分析，我们可以看到编译器在编译期对代码的优化特点：

    1 常量折叠，将编译期可以计算出的静态结果提前得出，将运行时计算开销降低为0。

    2 自动将字符串连接操作优化为StringBuilder类的append方法，以提高连接速度。

    3 对于调用方法动态生成的对象无法以常量折叠方式优化

    通过分析可以得知：

    1 为了提高可读性，将一个字符串常量分割为多个用于排版，不会影响运行效率；

    2 为了提高可读性，可以直接使用字符串连接符“+”，替换非线程同步的StringBuilder类，不会对运行时效率产生影响

3 使用单线程下使用StringBuffer反而会降低性能。猜测：1.5以前的版本可能没有区别，或使用StringBuffer更快，因为1.5才开始有StringBuilder类，也不知道1.5以前的编译器会不会使用StringBuffer替换字符串连接操作。

引申：
    存在域定义

    static final String a="a";

    static String b="b";

    static final String c=new String("c");

    反编译方法内部代码

    String name1 = "1"+a;

    String name2 = "2"+b;

    String name3 = "3"+c;

    效果如下：

   0:   ldc #2; //String 1a

   2:   astore_1

# 静态常量被直接折叠保存。

   3:   new #3; //class java/lang/StringBuilder

   6:   dup

   7:   invokespecial #4; //Method java/lang/StringBuilder."<init>":()V

   10: ldc #5; //String 2

   12: invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

   15: getstatic #7; //Field b:Ljava/lang/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: astore_2

# 变量未被折叠

   25: new #3; //class java/lang/StringBuilder

   28: dup

   29: invokespecial #4; //Method java/lang/StringBuilder."<init>":()V

   32: ldc #9; //String 3

   34: invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

   37: getstatic #10; //Field c:Ljava/lang/String;

   40: invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

   43: invokevirtual #8; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;

   46: astore_3

# 动态常量未被折叠保存

    分析：

    1 常量可以被认为运行时不可改变，所以编译时被以常量折叠方式优化。

    2 变量和动态生成的常量必须在运行时确定值，所以不能在编译期折叠优化

    结论：

    如class1 中某个地方直接引用了 class2中的某个final常量，则在编译时会将常量值记入class1的常量池中，或被常量折叠优化。如果class2修改了这个常量并重新编译，运行时class1中的值不会随之变动，而是使用旧的class2的值，导致程序出现不可预期的效果。

    所以建议通过动态赋值方式给常量赋值，如：

       final String str = new String(“str1”);

       final int one = new Integer(1);

    虽然增加了类初始化的时间，但可以保证final值所在class文件更新后其他class不用重新编译就可以使用新的值。（虽然随便修改final定义是不好的。。。）



附件：

用到以下2个类。

public class StringAdd{
    public static void main(String[] arg){
String name = "ab"+"cd";
String name1 = "ab" + new String("cd");
String name2 = "ab"+1+"cd";
String name3 = "ab"+"c" + new String("d");
StringBuffer name4 = new StringBuffer("e");
name4.append("f").append("g");
    }
}
public class StringAdd2{
static final String a="a";
static String b="b";
static final String c=new String("c");
public static void main(String[] args){
String name1 = "1"+a;
String name2 = "2"+b;
String name3 = "3"+c;
}
}

反编译结果如下：
Compiled from "StringAdd.java"
public class StringAdd extends java.lang.Object
SourceFile: "StringAdd.java"
minor version: 0
major version: 50
Constant pool:
const #1 = Method #21.#30; // java/lang/Object."<init>":()V
const #2 = String #31; // abcd
const #3 = class #32; // java/lang/StringBuilder
const #4 = Method #3.#30; // java/lang/StringBuilder."<init>":()V
const #5 = String #33; // ab
const #6 = Method #3.#34; // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
const #7 = class #35; // java/lang/String
const #8 = String #36; // cd
const #9 = Method #7.#37; // java/lang/String."<init>":(Ljava/lang/String;)V
const #10 = Method #3.#38; // java/lang/StringBuilder.toString:()Ljava/lang/String;
const #11 = String #39; // ab1cd
const #12 = String #40; // abc
const #13 = String #41; // d
const #14 = class #42; // java/lang/StringBuffer
const #15 = String #43; // e
const #16 = Method #14.#37; // java/lang/StringBuffer."<init>":(Ljava/lang/String;)V
const #17 = String #44; // f
const #18 = Method #14.#45; // java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;
const #19 = String #46; // g
const #20 = class #47; // StringAdd
const #21 = class #48; // java/lang/Object
const #22 = Asciz <init>;
const #23 = Asciz ()V;
const #24 = Asciz Code;
const #25 = Asciz LineNumberTable;
const #26 = Asciz main;
const #27 = Asciz ([Ljava/lang/String;)V;
const #28 = Asciz SourceFile;
const #29 = Asciz StringAdd.java;
const #30 = NameAndType #22:#23;// "<init>":()V
const #31 = Asciz abcd;
const #32 = Asciz java/lang/StringBuilder;
const #33 = Asciz ab;
const #34 = NameAndType #49:#50;// append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
const #35 = Asciz java/lang/String;
const #36 = Asciz cd;
const #37 = NameAndType #22:#51;// "<init>":(Ljava/lang/String;)V
const #38 = NameAndType #52:#53;// toString:()Ljava/lang/String;
const #39 = Asciz ab1cd;
const #40 = Asciz abc;
const #41 = Asciz d;
const #42 = Asciz java/lang/StringBuffer;
const #43 = Asciz e;
const #44 = Asciz f;
const #45 = NameAndType #49:#54;// append:(Ljava/lang/String;)Ljava/lang/StringBuffer;
const #46 = Asciz g;
const #47 = Asciz StringAdd;
const #48 = Asciz java/lang/Object;
const #49 = Asciz append;
const #50 = Asciz (Ljava/lang/String;)Ljava/lang/StringBuilder;;
const #51 = Asciz (Ljava/lang/String;)V;
const #52 = Asciz toString;
const #53 = Asciz ()Ljava/lang/String;;
const #54 = Asciz (Ljava/lang/String;)Ljava/lang/StringBuffer;;

{
public StringAdd();
LineNumberTable:
   line 1: 0

Code:
   Stack=1, Locals=1, Args_size=1
   0: aload_0
   1: invokespecial #1; //Method java/lang/Object."<init>":()V
   4: return
LineNumberTable:
   line 1: 0

public static void main(java.lang.String[]);
LineNumberTable:
   line 3: 0
   line 4: 3
   line 5: 31
   line 6: 34
   line 7: 63
   line 8: 74
   line 9: 87

Code:
   Stack=4, Locals=6, Args_size=1
   0: ldc #2; //String abcd
   2: astore_1
   3: new #3; //class java/lang/StringBuilder
   6: dup
   7: invokespecial #4; //Method java/lang/StringBuilder."<init>":()V
   10: ldc #5; //String ab
   12: invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   15: new #7; //class java/lang/String
   18: dup
   19: ldc #8; //String cd
   21: invokespecial #9; //Method java/lang/String."<init>":(Ljava/lang/String;)V
   24: invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   27: invokevirtual #10; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   30: astore_2
   31: ldc #11; //String ab1cd
   33: astore_3
   34: new #3; //class java/lang/StringBuilder
   37: dup
   38: invokespecial #4; //Method java/lang/StringBuilder."<init>":()V
   41: ldc #12; //String abc
   43: invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   46: new #7; //class java/lang/String
   49: dup
   50: ldc #13; //String d
   52: invokespecial #9; //Method java/lang/String."<init>":(Ljava/lang/String;)V
   55: invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   58: invokevirtual #10; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   61: astore 4
   63: new #14; //class java/lang/StringBuffer
   66: dup
   67: ldc #15; //String e
   69: invokespecial #16; //Method java/lang/StringBuffer."<init>":(Ljava/lang/String;)V
   72: astore 5
   74: aload 5
   76: ldc #17; //String f
   78: invokevirtual #18; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;
   81: ldc #19; //String g
   83: invokevirtual #18; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;
   86: pop
   87: return
LineNumberTable:
   line 3: 0
   line 4: 3
   line 5: 31
   line 6: 34
   line 7: 63
   line 8: 74
   line 9: 87

}

Compiled from "StringAdd2.java"
public class StringAdd2 extends java.lang.Object
SourceFile: "StringAdd2.java"
minor version: 0
major version: 50
Constant pool:
const #1 = Method #16.#32; // java/lang/Object."<init>":()V
const #2 = String #33; // 1a
const #3 = class #34; // java/lang/StringBuilder
const #4 = Method #3.#32; // java/lang/StringBuilder."<init>":()V
const #5 = String #35; // 2
const #6 = Method #3.#36; // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
const #7 = Field #15.#37; // StringAdd2.b:Ljava/lang/String;
const #8 = Method #3.#38; // java/lang/StringBuilder.toString:()Ljava/lang/String;
const #9 = String #39; // 3
const #10 = Field #15.#40; // StringAdd2.c:Ljava/lang/String;
const #11 = String #21; // b
const #12 = class #41; // java/lang/String
const #13 = String #22; // c
const #14 = Method #12.#42; // java/lang/String."<init>":(Ljava/lang/String;)V
const #15 = class #43; // StringAdd2
const #16 = class #44; // java/lang/Object
const #17 = Asciz a;
const #18 = Asciz Ljava/lang/String;;
const #19 = Asciz ConstantValue;
const #20 = String #17; // a
const #21 = Asciz b;
const #22 = Asciz c;
const #23 = Asciz <init>;
const #24 = Asciz ()V;
const #25 = Asciz Code;
const #26 = Asciz LineNumberTable;
const #27 = Asciz main;
const #28 = Asciz ([Ljava/lang/String;)V;
const #29 = Asciz <clinit>;
const #30 = Asciz SourceFile;
const #31 = Asciz StringAdd2.java;
const #32 = NameAndType #23:#24;// "<init>":()V
const #33 = Asciz 1a;
const #34 = Asciz java/lang/StringBuilder;
const #35 = Asciz 2;
const #36 = NameAndType #45:#46;// append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
const #37 = NameAndType #21:#18;// b:Ljava/lang/String;
const #38 = NameAndType #47:#48;// toString:()Ljava/lang/String;
const #39 = Asciz 3;
const #40 = NameAndType #22:#18;// c:Ljava/lang/String;
const #41 = Asciz java/lang/String;
const #42 = NameAndType #23:#49;// "<init>":(Ljava/lang/String;)V
const #43 = Asciz StringAdd2;
const #44 = Asciz java/lang/Object;
const #45 = Asciz append;
const #46 = Asciz (Ljava/lang/String;)Ljava/lang/StringBuilder;;
const #47 = Asciz toString;
const #48 = Asciz ()Ljava/lang/String;;
const #49 = Asciz (Ljava/lang/String;)V;

{
static final java.lang.String a;
Constant value: String a

static java.lang.String b;

static final java.lang.String c;

public StringAdd2();
LineNumberTable:
   line 1: 0

Code:
   Stack=1, Locals=1, Args_size=1
   0: aload_0
   1: invokespecial #1; //Method java/lang/Object."<init>":()V
   4: return
LineNumberTable:
   line 1: 0

public static void main(java.lang.String[]);
LineNumberTable:
   line 6: 0
   line 7: 3
   line 8: 25
   line 9: 47

Code:
   Stack=2, Locals=4, Args_size=1
   0: ldc #2; //String 1a
   2: astore_1
   3: new #3; //class java/lang/StringBuilder
   6: dup
   7: invokespecial #4; //Method java/lang/StringBuilder."<init>":()V
   10: ldc #5; //String 2
   12: invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   15: getstatic #7; //Field b:Ljava/lang/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: astore_2
   25: new #3; //class java/lang/StringBuilder
   28: dup
   29: invokespecial #4; //Method java/lang/StringBuilder."<init>":()V
   32: ldc #9; //String 3
   34: invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   37: getstatic #10; //Field c:Ljava/lang/String;
   40: invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   43: invokevirtual #8; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   46: astore_3
   47: return
LineNumberTable:
   line 6: 0
   line 7: 3
   line 8: 25
   line 9: 47

static {};
LineNumberTable:
   line 3: 0
   line 4: 5

Code:
   Stack=3, Locals=0, Args_size=0
   0: ldc #11; //String b
   2: putstatic #7; //Field b:Ljava/lang/String;
   5: new #12; //class java/lang/String
   8: dup
   9: ldc #13; //String c
   11: invokespecial #14; //Method java/lang/String."<init>":(Ljava/lang/String;)V
   14: putstatic #10; //Field c:Ljava/lang/String;
   17: return
LineNumberTable:
   line 3: 0
   line 4: 5

}

JVM编译期字符串联接优化分析

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