String

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String

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public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {
    /** The value is used for character storage. */
    private final char value[];

    /** Cache the hash code for the string */
    private int hash; // Default to 0

    public int hashCode() {
        int h = hash;
        if (h == 0 && value.length > 0) {
            char val[] = value;

            for (int i = 0; i < value.length; i++) {
                h = 31 * h + val[i];//32=2^5    左移5位减去自身+字符ASCII码值
            }
            hash = h;
        }
        return h;
    }
    public boolean equals(Object anObject) {
        if (this == anObject) {//同一个引用
            return true;
        }
        if (anObject instanceof String) {
            String anotherString = (String)anObject;
            int n = value.length;
            if (n == anotherString.value.length) {//长度相同（减少比较次数）
                char v1[] = value;
                char v2[] = anotherString.value;
                int i = 0;
                while (n-- != 0) {//。。。
                    if (v1[i] != v2[i])
                        return false;
                    i++;
                }
                return true;
            }
        }
        return false;
    }
}

String的存储

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NewUnsafeTest.java
NewUnsafeTest.class

String s=”abc”
存储在常量区？？？
String str=new String(“abc”)
存储在堆区

源代码

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public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {
    Unsafe unsafe = getUnsafe();
    final String s = "abc";
    String s1 = "abc";

    System.out.println(s==s1);
    //获取 String.Field:value
    Field valueInString = String.class.getDeclaredField("value");
    //获取 Field 偏移值
    long offset = unsafe.objectFieldOffset(valueInString);

    ////基址+偏移
    long base = unsafe.arrayBaseOffset(char[].class);
    long scale = unsafe.arrayIndexScale(char[].class);

    char[] values = (char[]) unsafe.getObject(s, offset);
    unsafe.putChar(values, base + scale * 2, 'd');

    System.out.println("s:" + s + " s1:" + s1);

    //将s的值改为 abc
    s1="abc";
    String s2 = "abc";
    String s3 = new String("abc");              // 源码：String(String original){this.value = original.value;}   --->  "abc"
    System.out.println(s==s1);
    System.out.println(s.equals(s1));
    System.out.println("s:" + s + " s1:" + s1 + "  s2:" + s2 + "  s3:" + s3);
    Field valueField=String.class.getDeclaredField("value");
    valueField.setAccessible(true);

    ///一个字符串在常量曲中一处内存
    System.out.println("c");
    System.out.println("c"==(s1.substring(2)));
    System.out.println(((char[])valueField.get(s1))[2]+"\t"+((char[])valueField.get("c"))[0]);
    System.out.println(((char[])valueField.get(s1))[2]==((char[])valueField.get("abcd"))[2]);

    ///"abc".equals(s1)  ---> 字符比较过程
    System.out.println(((char[])valueField.get(s1))[2]+"\t"+((char[])valueField.get("abc"))[2]);
    System.out.println(((char[])valueField.get(s1))[2]==((char[])valueField.get("abc"))[2]);
    System.out.println("d".equals(s1.substring(2)));


    ///由于字符串value值一般无法更改，内存中 常量池含有字符串"abc" 直接输出---->  "abd"  常量区数据改变
    System.out.println("abc");



    ////Arrays.copyOf(value, value.length);
    char[] ori=new char[]{'a','b','c'};
    String s4=new String(ori);
    System.out.println(s4);

    values= (char[]) unsafe.getObject(s4,offset);
    unsafe.putChar(values,base+scale*2,'d');
    System.out.println(s4);
    System.out.println(Arrays.toString(ori));
    unsafe.putChar(ori,base+scale*2,'d');
    System.out.println(Arrays.toString(ori));



    String s5 = "";
    values = (char[]) unsafe.getObject(s5, offset);
    System.out.println("values: "+Arrays.toString(values));

    // where is this in NewUnsfae.class ????????????????????????????????????????????
    values = ori;//----------------------------------------------------why-----  ???
    System.out.println(values.length);
    values = ori;//----------------------------------------------------why-----  ???
    System.out.println(values.length);
    values = ori;//----------------------------------------------------why-----  ???
    values = ori;//----------------------------------------------------why-----  ???
    values = ori;//----------------------------------------------------why-----  ???
    values = ori;//----------------------------------------------------why-----  ???
    values = ori;//----------------------------------------------------why-----  ???
    values = ori;//----------------------------------------------------why-----  ???
    values = ori;//----------------------------------------------------why-----  ???
    values = ori;//----------------------------------------------------why-----  ???
    values = ori;//----------------------------------------------------why-----  ???
    values = ori;//----------------------------------------------------why-----  ???
    values = ori;//----------------------------------------------------why-----  ???
    values = ori;//----------------------------------------------------why-----  ???
    System.out.println(values.length);
   //        System.arraycopy(ori, 0, values, 0, 3);
    System.out.println("s5: " + s5);
    System.out.println("values: " + Arrays.toString(values));
    System.out.println("ori: " + Arrays.toString(ori));
    unsafe.putChar(values, base + scale * 2, 'x');
    System.out.println("s5: " + s5);
    System.out.println("values: " + Arrays.toString(values));
    System.out.println("ori: " + Arrays.toString(ori));
    unsafe.putChar(ori, base + scale * 2, 'y');
    System.out.println("s5: " + s5);
    System.out.println("values: " + Arrays.toString(values));
    System.out.println("ori: " + Arrays.toString(ori));
}

private static Unsafe getUnsafe() {
    Unsafe unsafe = null;
    try {
        Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
        theUnsafe.setAccessible(true);
        unsafe = (Unsafe) theUnsafe.get(Unsafe.class);//or (Unsafe) theUnsafe.get(null);
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        return unsafe;
    }
}

Result：

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true                                        s==s1         //编译期替换  "abc" == s1                                
s:abc s1:abd
true                                        s==s1
true                                        s.equals(s1)
s:abc s1:abd  s2:abd  s3:abd
c                                           "c"
false                                       "c".equals(s1.substring(2)))  
d	c                                         ((char[])valueField.get(s1))[2]+"\t"+((char[])valueField.get("c"))[0]
false                                       ((char[])valueField.get(s1))[2]  ==   ((char[])valueField.get("c"))[0]
d	d                                         ((char[])valueField.get(s1))[2]+"\t"+((char[])valueField.get("abc"))[2]
true                                        ((char[])valueField.get(s1))[2]  == ((char[])valueField.get("abc"))[2]
true                                        "d".equals(s1.substring(2))
abd                                         new String("abc")
abc                                         new String( new char[]{'a','b','c'} )
abd                                         s4: unsafe 修改内存
[a, b, c]                                   ori: 内存不变
[a, b, d]                                   ori: unsafe修改内存

Process finished with exit code 0

s=abc而s1=abd

参见字节码文件NewUnsafeTest.class

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public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException, InstantiationException {
   Unsafe unsafe = getUnsafe();
   String s = "abc";
   String s1 = "abc";
   System.out.println("abc" == s1);
   Field valueInString = String.class.getDeclaredField("value");
   long offset = unsafe.objectFieldOffset(valueInString);
   long base = (long)unsafe.arrayBaseOffset(char[].class);
   long scale = (long)unsafe.arrayIndexScale(char[].class);
   char[] values = (char[])((char[])unsafe.getObject("abc", offset));
   unsafe.putChar(values, base + scale * 2L, 'd');
   System.out.println("s:abc s1:" + s1);
   s1 = "abc";
   String s2 = "abc";
   String s3 = new String("abc");
   System.out.println("abc" == s1);
   System.out.println("abc".equals(s1));
   System.out.println("s:abc s1:" + s1 + "  s2:" + s2 + "  s3:" + s3);
   Field valueField = String.class.getDeclaredField("value");
   valueField.setAccessible(true);
   System.out.println("c");
   System.out.println("c" == s1.substring(2));
   System.out.println(((char[])((char[])valueField.get(s1)))[2] + "\t" + ((char[])((char[])valueField.get("c")))[0]);
   System.out.println(((char[])((char[])valueField.get(s1)))[2] == ((char[])((char[])valueField.get("abcd")))[2]);
   System.out.println(((char[])((char[])valueField.get(s1)))[2] + "\t" + ((char[])((char[])valueField.get("abc")))[2]);
   System.out.println(((char[])((char[])valueField.get(s1)))[2] == ((char[])((char[])valueField.get("abc")))[2]);
   System.out.println("d".equals(s1.substring(2)));
   System.out.println("abc");
   char[] ori = new char[]{'a', 'b', 'c'};
   String s4 = new String(ori);
   System.out.println(s4);
   values = (char[])((char[])unsafe.getObject(s4, offset));
   unsafe.putChar(values, base + scale * 2L, 'd');
   System.out.println(s4);
   System.out.println(Arrays.toString(ori));
   unsafe.putChar(ori, base + scale * 2L, 'd');
   System.out.println("ori: " + Arrays.toString(ori));
   String s5 = "";
   values = (char[])((char[])unsafe.getObject(s5, offset));
   System.out.println("values: " + Arrays.toString(values));
   System.out.println(ori.length);
   System.out.println(ori.length);
   System.out.println(ori.length);
   System.out.println("s5: " + s5);
   System.out.println("values: " + Arrays.toString(ori));
   System.out.println("ori: " + Arrays.toString(ori));
   unsafe.putChar(ori, base + scale * 2L, 'x');
   System.out.println("s5: " + s5);
   System.out.println("values: " + Arrays.toString(ori));
   System.out.println("ori: " + Arrays.toString(ori));
   unsafe.putChar(ori, base + scale * 2L, 'y');
   System.out.println("s5: " + s5);
   System.out.println("values: " + Arrays.toString(ori));
   System.out.println("ori: " + Arrays.toString(ori));
}

private static Unsafe getUnsafe() {
   Unsafe unsafe = null;

   try {
       Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
       theUnsafe.setAccessible(true);
       unsafe = (Unsafe)theUnsafe.get(Unsafe.class);
       return unsafe;
   } catch (Exception var5) {
       var5.printStackTrace();
       return unsafe;
   } finally {
       ;
   }
}

“abc”==s1 & “abc”.equals(s1) –> true,true

Java.lang.String: Strings are constant; their values cannot be changed after they are created. String buffers support mutable strings. Because String objects are immutable they can be shared.

Because String objects are immutable they can be shared.
“abc” is shared……

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String s1="abc" //在内存中存储在常量区中  --->  更改内存：常量区 'c' ---> 'd'
字符串"abc"引用指向内存常量--->   "abd"

s3=”abd” & s4=”abc”

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public String(String original) {
    this.value = original.value;                    //指向同一块内存
    this.hash = original.hash;
}
public String(char value[]) {
    this.value = Arrays.copyOf(value, value.length);//新建一块内存存储当前值
}
public static char[] copyOf(char[] original, int newLength) {
    char[] copy = new char[newLength];
    System.arraycopy(original, 0, copy, 0,
                     Math.min(original.length, newLength));
    return copy;
}

反射修改常量值

不能修改的原因是编译期替换

避免编译期替换

  1. 构造方法中对常量赋值
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    class Test{
      private final String FINAL_STR;
      public Test(){
        this.FINAL_STR="FINAL";
      }
    }
  2. 三目运算符
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    class Test{
      private final String FINAL_STR = null == null ? "FINAL" :"";
      public Test(){
      }
    }
-------------再接再厉更进一步---------------