Java深入研究String字符串提及String字符串,我们更多的是用于文本的传输与存储，在JDK源码中也被申明为fi

提及String字符串,我们更多的是用于文本的传输与存储，在JDK源码中也被申明为final类型，同时也不属于Java中基本的数据类型,例如以直接双引号申明的常量String nameStr="Manna Yang";或者采用构造函数创建String nameStr=new String("Manna Yang");下面将逐步揭开其神秘面纱...

class字节码文件结构

在探究String字符串常量池之前，我们首先看下通过javap -v命令编译后的字节码

原始Java代码，通过javac编译为class,

public class TestString{
      private String testStr="Manna Yang";
      public static int TYPE=0;
      
      public static void main(String[] args){
            System.out.println("Manna Yang");
      }
}

编译后的字节码

Classfile /C:/Users/15971/Desktop/TestString.class
  Last modified 2019-9-18; size 566 bytes
  MD5 checksum 72f3c93ff8293c97a3da06775fa48ba0
  Compiled from "TestString.java"
public class TestString
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #8.#22         // java/lang/Object."<init>":()V
   #2 = String             #23            // Manna Yang
   #3 = Fieldref           #7.#24         // TestString.testStr:Ljava/lang/String;
   #4 = Fieldref           #25.#26        // java/lang/System.out:Ljava/io/PrintStream;
   #5 = Methodref          #27.#28        // java/io/PrintStream.println:(Ljava/lang/String;)V
   #6 = Fieldref           #7.#29         // TestString.TYPE:I
   #7 = Class              #30            // TestString
   #8 = Class              #31            // java/lang/Object
   #9 = Utf8               testStr
  #10 = Utf8               Ljava/lang/String;
  #11 = Utf8               TYPE
  #12 = Utf8               I
  #13 = Utf8               <init>
  #14 = Utf8               ()V
  #15 = Utf8               Code
  #16 = Utf8               LineNumberTable
  #17 = Utf8               main
  #18 = Utf8               ([Ljava/lang/String;)V
  #19 = Utf8               <clinit>
  #20 = Utf8               SourceFile
  #21 = Utf8               TestString.java
  #22 = NameAndType        #13:#14        // "<init>":()V
  #23 = Utf8               Manna Yang
  #24 = NameAndType        #9:#10         // testStr:Ljava/lang/String;
  #25 = Class              #32            // java/lang/System
  #26 = NameAndType        #33:#34        // out:Ljava/io/PrintStream;
  #27 = Class              #35            // java/io/PrintStream
  #28 = NameAndType        #36:#37        // println:(Ljava/lang/String;)V
  #29 = NameAndType        #11:#12        // TYPE:I
  #30 = Utf8               TestString
  #31 = Utf8               java/lang/Object
  #32 = Utf8               java/lang/System
  #33 = Utf8               out
  #34 = Utf8               Ljava/io/PrintStream;
  #35 = Utf8               java/io/PrintStream
  #36 = Utf8               println
  #37 = Utf8               (Ljava/lang/String;)V
{
  public static int TYPE;
    descriptor: I
    flags: ACC_PUBLIC, ACC_STATIC

  public TestString();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: aload_0
         5: ldc           #2                  // String Manna Yang
         7: putfield      #3                  // Field testStr:Ljava/lang/String;
        10: return
      LineNumberTable:
        line 1: 0
        line 2: 4

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #4    // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #2    // String Manna Yang
         5: invokevirtual #5    // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: return
      LineNumberTable:
        line 6: 0
        line 7: 8

  static {};
    descriptor: ()V
    flags: ACC_STATIC
    Code:
      stack=1, locals=0, args_size=0
         0: iconst_0
         1: putstatic     #6                  // Field TYPE:I
         4: return
      LineNumberTable:
        line 3: 0
}
SourceFile: "TestString.java"

Constant pool位置之前，依次是对当前编译的class、最后修改时间、MD5校验、Java的次要版本、主版本52(十进制的值,对应jdk 1.8，转换为16进制为34)、标记是否是public、是否调用超类构造方法
Constant pool位置以下,从 #1 - #37 对应常量池区域,存放方法签名以及定义的String字面值,例如#2对应#23 即是Java代码中private String testStr="Manna Yang"; #29 NameAndType 对应#11:#12 即是Java代码中的public static int TYPE=0;
方法签名类型如下

签名字符 | 方法类型 | | :-: | :-: | :-: | B | byte | C | char| D | double| F | float| I | int| J | long| L | 引用类型| S | short| Z | boolean| [ | 数组类型| V | Void类型|

6. 字节码指令

public TestString();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: aload_0
         5: ldc           #2                  // String Manna Yang
         7: putfield      #3                  // Field testStr:Ljava/lang/String;
        10: return
      LineNumberTable:
        line 1: 0
        line 2: 4

descriptor : 描述方法类型
flags : 描述修饰符
stack : 操作数堆栈大小
locals : 局部变量数大小
agrs_size : 方法参数个数
load类型指令常见的有aload,fload,iload,dload,此处aload_0表示将本地变量推送到栈顶，a表示引用类型，i\d\f分别对应基本类型，结构基本遵循 : 类型|动作
const类型指令常见有iconst,iconst,fconst,dconst,例如定义int testType=2;在父类构造方法中就会存在iconst_0(下划线后面为index,表示变量位置),表示将int型常量推送到栈顶;
ldc : 将int，float或String型常量从常量池中推送至栈顶
putfield : 赋值操作，对应还有getfield
return : 返回void,对应还有ireturn、freturn,表示返回int\float类型
invokespecial : 调用父类无参无返回值构造方法
putstatic : 静态变量赋值，对应还有getstatic

String字符串equals、hashcode、intern方法

1.了解上述字节码结构之后，再来看看常用的字符串比较

public boolean equals(Object anObject) {
        if (this == anObject) {
            return true;
        }
        if (anObject instanceof String) {
            String anotherString = (String) anObject;
            int n = length();
            if (n == anotherString.length()) {
                int i = 0;
                while (n-- != 0) {
                    if (charAt(i) != anotherString.charAt(i))
                            return false;
                    i++;
                }
                return true;
            }
        }
        return false;
    }

默认还是比较常量池引用地址是否相等，否则对比类型，接着调用charAt()逐个字符比较，下面举例一些常见的比较场景，加深理解

String testStr1="Manna Yang";
String testStr2=new String("Manna Yang");
String testStr3="Manna Yang";

System.out.println(testStr1 == testStr2);           //false
System.out.println(testStr1.equals(testStr2));      //true
System.out.println(testStr1 == testStr3);           //true
System.out.println(testStr1.equals(testStr3));      //true
按照jdk中equals方法，此时==对比为false(地址不一样)，则继续采用charAt方式逐个比较字符，new关键字创建的
对象存放在heap堆，双引号""申明的常量放在常量池，testStr2引用指向常量池"Manna Yang"字符地址

继续往下看 + 号的魅力

String testStr0 = new String("Test")+new String("Manna")+new String("Yang");编码后如下

0: new           #2             // class java/lang/StringBuilder
3: dup
4: invokespecial #3             // Method java/lang/StringBuilder."<init>":()V
7: new           #4             // class java/lang/String
10: dup
11: ldc           #5            // String Test
13: invokespecial #6            // Method java/lang/String."<init>":(Ljava/lang/String;)V
16: invokevirtual #7            //Method  java/lang/StringBuilder.append:
                                (Ljava/lang/String;)Ljava/lang/StringBuilder;
19: new           #4            // class java/lang/String
22: dup
23: ldc           #8            // String Manna
25: invokespecial #6            // Method java/lang/String."<init>":(Ljava/lang/String;)V
28: invokevirtual #7            //Method  java/lang/StringBuilder.append:
                                (Ljava/lang/String;)Ljava/lang/StringBuilder;
31: new           #4            // class java/lang/String
34: dup
35: ldc           #9            // String Yang
37: invokespecial #6            // Method java/lang/String."<init>":(Ljava/lang/String;)V
40: invokevirtual #7            //Method  java/lang/StringBuilder.append:
                                (Ljava/lang/String;)Ljava/lang/StringBuilder;
43: invokevirtual #10       //Method  java/lang/StringBuilder.toString:()Ljava/lang/String;
46: astore_1
47: return

在字节码中可以看到+号 StringBuilder对象也参与一次创建,然后调用父类初始化方法,接着调用append方法,最后再调用toString(),字节码中new的指令包含4次，ldc指令包含3次;实际上jdk优化后的+号,在处理字符串拼接时提供很大便利,例如String testStr1="Manna"+" Yang";那么在字节码里面已经拼接成一个字符串常量"Manna Yang";还有常见的在new String(""+"")这种方式,字符串也是会拼接,对应只new一次String对象;

2.继续看下hashcode, hash值(哈希)主要用于散列存储结构中确认对象的地址,像常用的HashMap\HashTable,如果两个对象相同则它们的hash值一定相同；反之hash值相同的两个对象不一定相同;在进行hash计算时我们期望hash值的碰撞越少越好，提高查询效率,下面看下String的hashCode()方法源码

public int hashCode() {
    int h = hash;
    final int len = length();
    if (h == 0 && len > 0) {
        for (int i = 0; i < len; i++) {
            h = 31 * h + charAt(i);
        }
        hash = h;
    }
    return h;
}

关于31这个系数我理解的更多是散列分布的更为均匀,产生hash碰撞的几率更小,在源码说明里面也有计算公式推导 : s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1],charAt字符数组中字符对应的value值为ASCII值,null的ASCII值为0;

3.关于String类中的intern(),源码方法里有详细注释,来源于jdk1.8

When the intern method is invoked, if the pool already contains a
* string equal to this {@code String} object as determined by
* the {@link #equals(Object)} method, then the string from the pool is
* returned. Otherwise, this {@code String} object is added to the
* pool and a reference to this {@code String} object is returned.

//源码方法
public native String intern();

String、StringBuffer、StringBuilder使用场景

字符串拼接效率,如果是字面常量拼接,则直接使用""+""+""这种方式,+号优化后只会生成一个对象,如果是字符串对象之间拼接,在多线程中使用时应采用StringBuffer,大部分方法线程安全;否则可使用StringBuilder,后两者StringBuffer、StringBuilder的扩容机制为array.length+16,均继承抽象父类AbstractStringBuilder中的构造函数,源码如下

AbstractStringBuilder(int var1) {
    this.value = new char[var1];
}
...

每次都是重新new,然后再进行array copy,建议在初始拼接时传入指定预计字符串长度值

Java深入研究String字符串

class字节码文件结构

String字符串equals、hashcode、intern方法

String、StringBuffer、StringBuilder使用场景

以上涉及JDK源码部分均来自 JDK 1.8