下面有段简单的代码
public class StringPlusTest{
public static void mAIn(String[] args) {
String s1 = "aaa";
String s2 = "bbb";
String s = "ccc" +s1 + s2 +"ddd";
}}
使用 javap -c .\StringPlusTest.class反编译一下，得到
Compiled from "StringPlusTest.java"public class com.epoint.codetuning.test.StringPlusTest {
public com.epoint.codetuning.test.StringPlusTest();
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 #2 // String aaa
2: astore_1 3: ldc #3 // String bbb
5: astore_2 6: new #4 // class java/lang/StringBuilder
9: dup 10: invokespecial #5 // Method java/lang/StringBuilder."<init>":()V
13: ldc #6 // String ccc
15: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
18: aload_1 19: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
22: aload_2 23: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
26: ldc #8 // String ddd
28: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
31: invokevirtual #9 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
34: astore_3 35: return}
其中，
ldc 把常量池中的项压入栈 astore_1 将引用类型或returnAddress类型值存入局部变量1 astore_2 将引用类型或returnAddress类型值存入局部变量2 new 创建一个新对象 dup 复制栈顶部一个字长内容 invokespecial 根据编译时类型来调用实例方法 aload_1 从局部变量1中装载引用类型值 aload_2 从局部变量2中装载引用类型值 astore_3 将引用类型或returnAddress类型值存入局部变量3
更多指令见JVM指令手册
对于Java来说，这段代码原理上应该是：
public class StringPlusTest{
public static void main(String[] args) {
String s1 = "aaa";
String s2 = "bbb";
String s = new StringBuilder().append("ccc").append(s1).append(s2).append("ddd").toString();
}}
由此可见，Java中使用“+”拼接字符串的实现原理是通过建立临时StringBuilder对象调用append和toString方法实现。
对上面代码做一些修改
public class StringPlusTest{
public static void main(String[] args) {
String s1 = "aaa";
String s2 = null;
String s = "ccc" +s1 + s2 +"ddd";
System.out.println(s);
}}
结果是什么？
运行可以得到如下：
cccaaanullddd
查看StringBuilder源码
@Overridepublic StringBuilder append(String str) {
super.append(str);
return this;}
该方法调用继承父类AbstractStringBuilder的方法，再去父类中查看
public AbstractStringBuilder append(String str) {
if (str == null)
return appendNull();
int len = str.length();
ensureCapacityInternal(count + len);
str.getChars(0, len, value, count);
count += len;
return this;}private AbstractStringBuilder appendNull() {
int c = count;
ensureCapacityInternal(c + 4);
final char[] value = this.value;
value[c++] = 'n';
value[c++] = 'u';
value[c++] = 'l';
value[c++] = 'l';
count = c;
return this;}
appendNull方法简单来说就是容量+4，追加null字符串。
所以结果为”cccaaanullddd”。
