String类 "+"的使用运行原理

来源：互联网发布：linux top cpu 编辑：程序博客网时间：2024/05/04 11:33

public class Test{

public String s5 = "d";

public Integer s8 =1;

public String s7 = new String("g");

public void x()

{

String s5 = "e";

}

public static void y()

{

String s6 = "f";

}

public static void main(String[] args){

String s1 = "a";

String s2 = "b";

String s3 = "c";

String h7 = s1 + s2 + s3;

String h8 = "a"+"b"+"c";

String h1 = s1+"b"+"c";

String h9 =s1;

h9 = h9+"b"+"c";

String h10 =s1;

h10 += "b"+"c";

String h3 = s1+s2;

String h4 = h3+s3;

String h5 = "a"+"b";

String h6 = h3+s3;

StringBuilder result = new StringBuilder();

String s4 = result.append(s1).append(s2).append(s3).toString();

}

字节码结果如下：

public static void main(java.lang.String[]);

flags: ACC_PUBLIC, ACC_STATIC

Code:

stack=2, locals=15, args_size=1

0: ldc #12 // String a

2: astore_1

3: ldc #13 // String b

5: astore_2

6: ldc #14 // String c

8: astore_3

9: new #15 // class java/lang/StringBuilder

12: dup

13: invokespecial #16 // Method java/lang/StringBuilder. "<init>":()V

16: aload_1

17: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

20: aload_2

21: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

24: aload_3

25: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

28: invokevirtual #18 // Method java/lang/StringBuilder. toString:()Ljava/lang/String;

31: astore 4

33: ldc #19 // String abc

35: astore 5

37: new #15 // class java/lang/StringBuilder

40: dup

41: invokespecial #16 // Method java/lang/StringBuilder. "<init>":()V

44: aload_1

45: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

48: ldc #13 // String b

50: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

53: ldc #14 // String c

55: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

58: invokevirtual #18 // Method java/lang/StringBuilder. toString:()Ljava/lang/String;

61: astore 6

63: aload_1

64: astore 7

66: new #15 // class java/lang/StringBuilder

69: dup

70: invokespecial #16 // Method java/lang/StringBuilder. "<init>":()V

73: aload 7

75: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

78: ldc #13 // String b

80: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

83: ldc #14 // String c

85: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

88: invokevirtual #18 // Method java/lang/StringBuilder. toString:()Ljava/lang/String;

91: astore 7

93: aload_1

94: astore 8

96: new #15 // class java/lang/StringBuilder

99: dup

100: invokespecial #16 // Method java/lang/StringBuilder. "<init>":()V

103: aload 8

105: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

108: ldc #20 // String bc

110: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

113: invokevirtual #18 // Method java/lang/StringBuilder. toString:()Ljava/lang/String;

116: astore 8

118: new #15 // class java/lang/StringBuilder

121: dup

122: invokespecial #16 // Method java/lang/StringBuilder. "<init>":()V

125: aload_1

126: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

129: aload_2

130: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

133: invokevirtual #18 // Method java/lang/StringBuilder. toString:()Ljava/lang/String;

136: astore 9

138: new #15 // class java/lang/StringBuilder

141: dup

142: invokespecial #16 // Method java/lang/StringBuilder. "<init>":()V

145: aload 9

147: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

150: aload_3

151: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

154: invokevirtual #18 // Method java/lang/StringBuilder. toString:()Ljava/lang/String;

157: astore 10

159: ldc #21 // String ab

161: astore 11

163: new #15 // class java/lang/StringBuilder

166: dup

167: invokespecial #16 // Method java/lang/StringBuilder. "<init>":()V

170: aload 9

172: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

175: aload_3

176: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

179: invokevirtual #18 // Method java/lang/StringBuilder. toString:()Ljava/lang/String;

182: astore 12

184: new #15 // class java/lang/StringBuilder

187: dup

188: invokespecial #16 // Method java/lang/StringBuilder. "<init>":()V

191: astore 13

193: aload 13

195: aload_1

196: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

199: aload_2

200: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

203: aload_3

204: invokevirtual #17 // Method java/lang/StringBuilder. append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

207: invokevirtual #18 // Method java/lang/StringBuilder. toString:()Ljava/lang/String;

210: astore 14

212: return

LineNumberTable:

line 14: 0

line 15: 3

line 16: 6

line 19: 9

line 21: 33

line 23: 37

line 26: 63

line 27: 66

line 29: 93

line 30: 96

line 32: 118

line 33: 138

line 35: 159

line 36: 163

line 39: 184

line 40: 193

line 42: 212

}

结论：对于+号的使用主要分为三类：常量和常量，常量和变量，变量和变量。

常量和常量：不需要调用StingBuilder类的append方法。编译器优化直接创建常量。

常量和变量：需要调用StringBuilder。一行+语句需要调用一次StringBuilder类，而其中的append方法调用次数取决一行+里面的变量和常量的数量。但是有一种特殊的形式例如 String h10 =s1; h10 += "b"+"c"; 由于隐含h10参与运算，所以先append h10,接着编译器优化直接创建bc然后再append bc 所以调用了两次append。

变量和变量：需要调用StringBuilder类。一行+语句需要调用一次StringBuilder类，而其中的append方法调用次数取决一行+里面的变量数量。

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