Java深入研究String字符串 Detail - android,java,jdk8,源碼學習 MannaYang Blog

提及String字符串,我們更多的是用於文本的傳輸與存儲，在JDK源碼中也被申明為final類型，同時也不屬於Java中基本的數據類型,例如以直接雙引號申明的常量String nameStr="Manna Yang";或者採用構造函數創建String nameStr=new String("Manna Yang");下面將逐步揭開其神秘面紗...

class字節碼文件結構

在探究String字符串常量池之前，我們首先看下通過javap -v命令編譯後的字節碼

原始Java代碼，通過javac編譯為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");
   }
}

編譯後的字節碼

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，依次是對當前編譯的class、最後修改時間、MD5校驗、Java的次要版本、主版本52(十進制的值,對應jdk 1.8，轉換為16進製為34)、標記是否是public、是否調用超類構造方法
Constant pool位置以下,從 #1 - #37 對應常量池區域,存放方法簽名以及定義的String字面值,例如#2對應#23 即是Java代碼中private String testStr="Manna Yang"; #29 NameAndType 對應#11:#12 即是Java代碼中的public static int TYPE=0;
方法簽名類型如下
<center>

簽名字符	方法類型
B	byte
C	char
D	double
F	float
I	int
J	long
L	引用類型
S	short
Z	boolean
[	數組類型
V	Void類型

</center>

字節碼指令

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 : 描述方法類型
flags : 描述修飾符
stack : 操作數堆棧大小
locals : 局部變量數大小
agrs_size : 方法參數個數
load類型指令
常見的有aload,fload,iload,dload,此處aload_0表示將本地變量推送到棧頂，a表示引用類型，i\d\f分別對應基本類型，結構基本遵循 : 類型|動作
const類型指令
常見有iconst,iconst,fconst,dconst,例如定義int testType=2;在父類構造方法中就會存在iconst_0(下劃線後面為index,表示變量位置),表示將int型常量推送到棧頂;
ldc : 將int，float或String型常量從常量池中推送至棧頂
putfield : 賦值操作，對應還有getfield
return : 返回void,對應還有ireturn、freturn,表示返回int\float類型
invokespecial : 調用父類無參無返回值構造方法
putstatic : 靜態變量賦值，對應還有getstatic

<br/>

String字符串equals、hashcode、intern方法

1.瞭解上述字節碼結構之後，再來看看常用的字符串比較

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;
    }

默認還是比較常量池引用地址是否相等，否則對比類型，接着調用charAt()逐個字符比較，下面舉例一些常見的比較場景，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，此時==對比為false(地址不一樣)，則繼續採用charAt方式逐個比較字符，new關鍵字創建的
對象存放在heap堆，雙引號""申明的常量放在常量池，testStr2引用指向常量池"Manna Yang"字符地址

繼續往下看 + 號的魅力

String testStr0 = new String("Test")+new String("Manna")+new String("Yang");編碼後如下

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

在字節碼中可以看到+號 StringBuilder對象也參與一次創建,然後調用父類初始化方法,接着調用append方法,最後再調用toString(),字節碼中new的指令包含4次，ldc指令包含3次;實際上jdk優化後的+號,在處理字符串拼接時提供很大便利,例如String testStr1="Manna"+" Yang";那麼在字節碼裏面已經拼接成一個字符串常量"Manna Yang";還有常見的在new String(""+"")這種方式,字符串也是會拼接,對應只new一次String對象;

2.繼續看下hashcode, hash值(哈希)主要用於散列存儲結構中確認對象的地址,像常用的HashMap\HashTable,如果兩個對象相同則它們的hash值一定相同；反之hash值相同的兩個對象不一定相同;在進行hash計算時我們期望hash值的碰撞越少越好，提高查詢效率,下面看下String的hashCode()方法源碼

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;
}

關於31這個係數我理解的更多是散列分佈的更為均勻,產生hash碰撞的機率更小,在源碼説明裏面也有計算公式推導 : s[0]31^(n-1) + s[1]31^(n-2) + ... + s[n-1],charAt字符數組中字符對應的value值為ASCII值,null的ASCII值為0;

3.關於String類中的intern(),源碼方法裏有詳細註釋,來源於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.

//源碼方法
public native String intern();

String、StringBuffer、StringBuilder使用場景

字符串拼接效率,如果是字面常量拼接,則直接使用""+""+""這種方式,+號優化後只會生成一個對象,如果是字符串對象之間拼接,在多線程中使用時應採用StringBuffer,大部分方法線程安全;否則可使用StringBuilder,後兩者StringBuffer、StringBuilder的擴容機制為array.length+16,均繼承抽象父類AbstractStringBuilder中的構造函數,源碼如下

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

每次都是重新new,然後再進行array copy,建議在初始拼接時傳入指定預計字符串長度值

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Java深入研究String字符串 - Stories Detail

class字節碼文件結構

String字符串equals、hashcode、intern方法

String、StringBuffer、StringBuilder使用場景

以上涉及JDK源碼部分均來自 JDK 1.8

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