Java: why the body's circulation to change the string concatenation worth StringBuilder.append?

Two code examples:

1. Example 1:

   public static void main(String[] args) {
   	String testStr = "test";
   	String rst = testStr + 1 + "a" + "pig" + 2;
   	System.out.println(rst);
   }
   复制代码

2. Example 2:

   public static void main(String[] args) {
   	String testStr = "";
   	for (int i = 0; i < 10; i++) {
   		testStr += i + "test";
   	}
   	System.out.println(testStr);
   }
   复制代码

analysis:

1. With javap decompile 例1obtain the following bytecode class file code represents:

   public static Method main:"([Ljava/lang/String;)V"
   stack 3 locals 3
   {
   	ldc	String "test";
   	astore_1;
   	new	class java/lang/StringBuilder;
   	dup;
   	aload_1;
   	invokestatic	Method java/lang/String.valueOf:"(Ljava/lang/Object;)Ljava/lang/String;";
   	invokespecial	Method java/lang/StringBuilder."<init>":"(Ljava/lang/String;)V";
   	iconst_1;
   	invokevirtual	Method java/lang/StringBuilder.append:"(I)Ljava/lang/StringBuilder;";
   	ldc	String "a";
   	invokevirtual	Method java/lang/StringBuilder.append:"(Ljava/lang/String;)Ljava/lang/StringBuilder;";
   	ldc	String "pig";
   	invokevirtual	Method java/lang/StringBuilder.append:"(Ljava/lang/String;)Ljava/lang/StringBuilder;";
   	iconst_2;
   	invokevirtual	Method java/lang/StringBuilder.append:"(I)Ljava/lang/StringBuilder;";
   	invokevirtual	Method java/lang/StringBuilder.toString:"()Ljava/lang/String;";
   	astore_2;
   	getstatic	Field java/lang/System.out:"Ljava/io/PrintStream;";
   	aload_2;
   	invokevirtual	Method java/io/PrintStream.println:"(Ljava/lang/String;)V";
   	return;
   	
   }
   复制代码

   Javac automatically seen such a continuous string concatenation compiled into StringBuilder.append, and therefore do not have to deliberately written StringBuilder.append form when writing code;
2. With javap decompile 例2obtain the following bytecode class file code represents:

   public static Method main:"([Ljava/lang/String;)V"
   stack 3 locals 3
   {
   		ldc	String "";              // 栈上来个空字符串
   		astore_1;                       // 栈顶元素弹出，放到局部变量1
   		iconst_0;                       // 栈上来个0
   		istore_2;                       // 栈顶元素弹出，放到局部变量2
   		goto	L35;                    // 跳转到35行，此时栈空
   	L8:	stack_frame_type append;
   		locals_map class java/lang/String, int;
   		new	class java/lang/StringBuilder;                                                    // 栈上来个StringBuilder对象
   		dup;                                                                                      // 复制栈顶对象，也压入栈中(StringBuilder对象)
   		aload_1;                                                                                  // 局部变量1压入栈中
   		invokestatic	Method java/lang/String.valueOf:"(Ljava/lang/Object;)Ljava/lang/String;"; // 栈顶元素强转String
   		invokespecial	Method java/lang/StringBuilder."<init>":"(Ljava/lang/String;)V";          // 弹出栈顶2个元素，调用StringBuilder构造函数，剩下一个刚才复制的、现在已经初始化好的StringBuilder在栈顶
   		iload_2;                                                                                  // 局部变量2压入栈中:数字0
   		invokevirtual	Method java/lang/StringBuilder.append:"(I)Ljava/lang/StringBuilder;";     // 数字0 append进栈顶StringBuilder,返回值StringBuilder放回栈顶
   		ldc	String "test";                                                                    // 常量"test"压入栈中
   		invokevirtual	Method java/lang/StringBuilder.append:"(Ljava/lang/String;)Ljava/lang/StringBuilder;"; // “test”append进栈顶StringBuilder，返回值StringBuilder放回栈顶
   		invokevirtual	Method java/lang/StringBuilder.toString:"()Ljava/lang/String;";                        // 栈顶StringBuilder弹出，toString，String结果放入栈顶
   		astore_1;                                                                                              // String结果弹出放进局部变量1
   		iinc	2, 1;                                                                                          // 2号局部变量++
   	L35:	stack_frame_type same;
   		iload_2;                                                            // 2号局部变量放到栈顶
   		bipush	10;                                                         // 常量10压栈，作为栈顶
   		if_icmplt	L8;                                                 // 弹出栈中前两个数，如果栈中第二个数比栈顶元素小，就跳到第8行，否则继续往下
   		getstatic	Field java/lang/System.out:"Ljava/io/PrintStream;"; // PrintStream对象压入栈顶
   		aload_1;                                                                    // 局部变量1压入栈顶(字符串)
   		invokevirtual	Method java/io/PrintStream.println:"(Ljava/lang/String;)V"; // 弹出栈中2个数，以栈中第二个元素作为参数1(即this)，栈顶元素作为参数2，调用println方法
   		return;
   	
   }
   复制代码

   A little long, a little I added some comments to read: javac still visible automatically cycle the body's string concatenation automatically compiled into StringBuilder.append, but please note that new class java/lang/StringBuilder;this directive appears in the L8label and L35back if_icmplt L8;between the jump instruction , that is to say: it circulates inside the body - again that is - the code is optimized for each cycle will again become a new StringBuilder do string concatenation, the number of loops, the number of StringBuilder object is created ; although this may not be a big deal of overhead, but it is still a programmer to write by hand the StringBuilder outside the loop, then the loop body using only append more cost-effective.

to sum up

Since modern javac compiler can automatically compiled to form StringBuilder.append string concatenation, and so usually the direct writing '+' sign to splice, but if the string needs to be spliced ​​through a cycle, preferably the explicit StringBuilder then create circulation in vivo use, in order to avoid repeated StringBuilder object created in vitro cycle