Java的String理解

String对象方式的创建方式

   public static void main(String[] args) {
        //1.new的方式创建
        String s = new String("abc");
        //2. 字面量字符串赋值
        String s1 = "bcd";
        //3. 多个字面量字符串拼接
        String s2 = "abc" + "de";
        //4. 含有变量的拼接
        String s3 = s1 + "de";
    }

查看main的字节码

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=3, locals=5, args_size=1
         0: new           #2                  // class java/lang/String
         3: dup
         4: ldc           #3                  // String abc
         6: invokespecial #4                  // Method java/lang/String."<init>":(Ljava/lang/String;)V
         9: astore_1
        10: ldc           #5                  // String bcd
        12: astore_2
        13: ldc           #6                  // String abcde
        15: astore_3
        16: new           #7                  // class java/lang/StringBuilder
        19: dup
        20: invokespecial #8                  // Method java/lang/StringBuilder."<init>":()V
        23: aload_2
        24: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        27: ldc           #10                 // String de
        29: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        32: invokevirtual #11                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        35: astore        4
        37: return
      LineNumberTable:
        line 12: 0
        line 14: 10
        line 16: 13
        line 18: 16
        line 19: 37
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      38         0  args         [Ljava/lang/String;
           10      28        1     s           Ljava/lang/String;
           13      25        2    s1          Ljava/lang/String;
           16      22        3    s2          Ljava/lang/String;
           37       1         4    s3          Ljava/lang/String;

//1.new的方式创建
String s = new String("abc");
创建的方式,可以看到
0: new #2 // class java/lang/String
这是在堆上创建了一个对象

//2. 字面量字符串赋值
String s1 = "bcd";
//3. 多个字面量字符串拼接
String s2 = "abc" + "de";
10: ldc #5 // String bcd
12: astore_2
13: ldc #6 // String abcde
15: astore_3
对象s2,s3直接从常量池的取值

ldc 表示从常量池提取数据并压入操作数栈

abcde并没有直接完整的形式出现,但是通过下面的常量池是可以发现abcde是存在常量池的
#6 = String #37 // abcde

所以: 若字符串通过字符串字面量的方式生成,是字符串的对象是从常量池中获取的.

1.必须全部是字符串字面量 2.可以有多个,单个字面量赋值是多个的特例

//4. 含有变量的拼接
String s3 = s1 + "de";

       16: new           #7                  // class java/lang/StringBuilder
        19: dup
        20: invokespecial #8                  // Method java/lang/StringBuilder."<init>":()V
        23: aload_2
        24: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        27: ldc           #10                 // String de
        29: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        32: invokevirtual #11                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;

通过字节码发现只要有(非fianl的)变量的参与拼接,那么jvm会通过新建StringBuilder对象的方式来创建
通过StringBuilder.append()的方法追踪源码,发现是不会共享String的char数组的.
若在s1前面添加final,
//2. 字面量字符串赋值
final String s1 = "bcd";
//4. 含有final变量的拼接
String s3 = s1 + "de";
那么等价于String s3 = "bcd" + "de";

继续变:
String s = new String("abc");
final String s1 = s;
String s3 = s1 + "de";
此时就算s1是final 的,但是因为s不是final 的很有可能变动,所以s1,s3都是通过在堆上新建StringBuilder对象生成字符串对象

对应的常量池

Constant pool:
   #1 = Methodref          #13.#32        // java/lang/Object."<init>":()V
   #2 = Class              #33            // java/lang/String
   #3 = String             #34            // abc
   #4 = Methodref          #2.#35         // java/lang/String."<init>":(Ljava/lang/String;)V
   #5 = String             #36            // bcd
   #6 = String             #37            // abcde
   #7 = Class              #38            // java/lang/StringBuilder
   #8 = Methodref          #7.#32         // java/lang/StringBuilder."<init>":()V
   #9 = Methodref          #7.#39         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #10 = String             #40            // de
  #11 = Methodref          #7.#41         // java/lang/StringBuilder.toString:()Ljava/lang/String;
  #12 = Class              #42            // com/cqs/learning/java/StringDemo
  #13 = Class              #43            // java/lang/Object
  #14 = Utf8               <init>
  #15 = Utf8               ()V
  #16 = Utf8               Code
  #17 = Utf8               LineNumberTable
  #18 = Utf8               LocalVariableTable
  #19 = Utf8               this
  #20 = Utf8               Lcom/cqs/learning/java/StringDemo;
  #21 = Utf8               main
  #22 = Utf8               ([Ljava/lang/String;)V
  #23 = Utf8               args
  #24 = Utf8               [Ljava/lang/String;
  #25 = Utf8               s
  #26 = Utf8               Ljava/lang/String;
  #27 = Utf8               s1
  #28 = Utf8               s2
  #29 = Utf8               s3
  #30 = Utf8               SourceFile
  #31 = Utf8               StringDemo.java
  #32 = NameAndType        #14:#15        // "<init>":()V
  #33 = Utf8               java/lang/String
  #34 = Utf8               abc
  #35 = NameAndType        #14:#44        // "<init>":(Ljava/lang/String;)V
  #36 = Utf8               bcd
  #37 = Utf8               abcde
  #38 = Utf8               java/lang/StringBuilder
  #39 = NameAndType        #45:#46        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #40 = Utf8               de
  #41 = NameAndType        #47:#48        // toString:()Ljava/lang/String;
  #42 = Utf8               com/cqs/learning/java/StringDemo
  #43 = Utf8               java/lang/Object
  #44 = Utf8               (Ljava/lang/String;)V
  #45 = Utf8               append
  #46 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
  #47 = Utf8               toString
  #48 = Utf8               ()Ljava/lang/String;