大家都知道String+String会开销额外的系统资源，粗略的原因是String是不可变类，每一步操作都会返回新的String变量，占用空间及时间。
其实我的理解不是这样的，我们来看看String+的底层实现。
测试案例
public static void main(String[] args) {
String a = "a";
StringBuilder b = new StringBuilder("b");
StringBuffer c = new StringBuffer("c");
long star = System.currentTimeMillis();
for(int i=0;i<100000;i++){
a+="a";
}
long end = System.currentTimeMillis();
System.out.println("String:"+(end-star));
star = System.currentTimeMillis();
for(int i=0;i<100000;i++){
b.append("b");
}
end = System.currentTimeMillis();
System.out.println("StringBuilder:"+(end-star));
star = System.currentTimeMillis();
for(int i=0;i<100000;i++){
c.append("c");
}
end = System.currentTimeMillis();
System.out.println("StringBuffer:"+(end-star));
}
测试结果
String:5355
StringBuilder:6
StringBuffer:9
测试结论
String+ 确实占用了太多的资源，处理效率非常低下。StringBuilder比StringBuffer更新了同步方法，性能有所提升。
原因分析
我用javap -verbose 查看已经编译好的class文件发现：
public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=6, locals=9, args_size=1
0: ldc #16 // String a
2: astore_1
3: new #18 // class java/lang/StringBuilder
6: dup
7: ldc #20 // String b
9: invokespecial #22 // Method java/lang/StringBuilder.
<init>":(Ljava/lang/String;)V
12: astore_2
13: new #25 // class java/lang/StringBuffer
16: dup
17: ldc #27 // String c
19: invokespecial #29 // Method java/lang/StringBuffer."
init>":(Ljava/lang/String;)V
22: astore_3
23: invokestatic #30 // Method java/lang/System.current
imeMillis:()J
26: lstore 4
28: iconst_0
29: istore 6
31: goto 57
34: new #18 // class java/lang/StringBuilder
37: dup
38: aload_1
39: invokestatic #36 // Method java/lang/String.valueOf
(Ljava/lang/Object;)Ljava/lang/String;
42: invokespecial #22 // Method java/lang/StringBuilder.
<init>":(Ljava/lang/String;)V
45: ldc #16 // String a
47: invokevirtual #42 // Method java/lang/StringBuilder.
ppend:(Ljava/lang/String;)Ljava/lang/StringBuilder;
50: invokevirtual #46 // Method java/lang/StringBuilder.
oString:()Ljava/lang/String;
53: astore_1
54: iinc 6, 1
57: iload 6
59: ldc #50 // int 100000
61: if_icmplt 34
64: invokestatic #30 // Method java/lang/System.current
imeMillis:()J
67: lstore 6
69: getstatic #51 // Field java/lang/System.out:Ljav
/io/PrintStream;
72: new #18 // class java/lang/StringBuilder
75: dup
76: ldc #55 // String String:
78: invokespecial #22 // Method java/lang/StringBuilder.
<init>":(Ljava/lang/String;)V
81: lload 6
83: lload 4
85: lsub
86: invokevirtual #57 // Method java/lang/StringBuilder.
ppend:(J)Ljava/lang/StringBuilder;
89: invokevirtual #46 // Method java/lang/StringBuilder.
oString:()Ljava/lang/String;
92: invokevirtual #60 // Method java/io/PrintStream.prin
ln:(Ljava/lang/String;)V
95: invokestatic #30 // Method java/lang/System.current
imeMillis:()J
98: lstore 4
100: iconst_0
101: istore 8
103: goto 116
106: aload_2
107: ldc #20 // String b
109: invokevirtual #42 // Method java/lang/StringBuilder.
ppend:(Ljava/lang/String;)Ljava/lang/StringBuilder;
112: pop
113: iinc 8, 1
116: iload 8
118: ldc #50 // int 100000
120: if_icmplt 106
123: invokestatic #30 // Method java/lang/System.current
imeMillis:()J
126: lstore 6
128: getstatic #51 // Field java/lang/System.out:Ljav
/io/PrintStream;
131: new #18 // class java/lang/StringBuilder
134: dup
135: ldc #65 // String StringBuilder:
137: invokespecial #22 // Method java/lang/StringBuilder.
<init>":(Ljava/lang/String;)V
140: lload 6
142: lload 4
144: lsub
145: invokevirtual #57 // Method java/lang/StringBuilder.
ppend:(J)Ljava/lang/StringBuilder;
148: invokevirtual #46 // Method java/lang/StringBuilder.
oString:()Ljava/lang/String;
151: invokevirtual #60 // Method java/io/PrintStream.prin
ln:(Ljava/lang/String;)V
154: invokestatic #30 // Method java/lang/System.current
imeMillis:()J
157: lstore 4
159: iconst_0
160: istore 8
162: goto 175
165: aload_3
166: ldc #27 // String c
168: invokevirtual #67 // Method java/lang/StringBuffer.a
pend:(Ljava/lang/String;)Ljava/lang/StringBuffer;
171: pop
172: iinc 8, 1
175: iload 8
177: ldc #50 // int 100000
179: if_icmplt 165
182: invokestatic #30 // Method java/lang/System.current
imeMillis:()J
185: lstore 6
187: getstatic #51 // Field java/lang/System.out:Ljav
/io/PrintStream;
190: new #18 // class java/lang/StringBuilder
193: dup
194: ldc #70 // String StringBuffer:
196: invokespecial #22 // Method java/lang/StringBuilder.
<init>":(Ljava/lang/String;)V
199: lload 6
201: lload 4
203: lsub
204: invokevirtual #57 // Method java/lang/StringBuilder.
ppend:(J)Ljava/lang/StringBuilder;
207: invokevirtual #46 // Method java/lang/StringBuilder.
oString:()Ljava/lang/String;
210: invokevirtual #60 // Method java/io/PrintStream.prin
ln:(Ljava/lang/String;)V
213: return
从编译的代码来看，String+的准确操作是：
new StringBuilder()
new String.valueof()
StringBuilder.<init>
StringBuilder.append()
StringBuilder.toString()
而StringBuilder的准确操作是：
StringBuilder.append()
而StringBuffer()的准确操作是：
StringBuffer.append
这样就可以理解为什么效率差这么多了！
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