String 的概述
为什么 JDK9 改变了 String 的结构
官方文档
http://openjdk.java.net/jeps/254
为什么改为 byte[] 存储?
// 之前
private final char value[];
// 之后
private final byte[] value
String 的基本特征
String:代表不可变的字符序列。简称:不可变性。
通过字面量的方式(区别于new)给一个字符串赋值,此时的字符串值声明在字符串常量池中。
当对字符串重新赋值时,需要重写指定内存区域赋值,不能使用原有的value进行赋值
@Test
public void test1() {
String s1 = "abc";//字面量定义的方式,"abc"存储在字符串常量池中
String s2 = "abc";
s1 = "hello";
System.out.println(s1 == s2);//判断地址:false
System.out.println(s1);//hello
System.out.println(s2);//abc
}
0 ldc #2 <abc>
2 astore_1
3 ldc #2 <abc>
5 astore_2
6 ldc #3 <hello>
8 astore_1
9 getstatic #4 <java/lang/System.out>
12 aload_1
13 aload_2
14 if_acmpne 21 (+7)
17 iconst_1
18 goto 22 (+4)
21 iconst_0
22 invokevirtual #5 <java/io/PrintStream.println>
25 getstatic #4 <java/lang/System.out>
28 aload_1
29 invokevirtual #6 <java/io/PrintStream.println>
32 getstatic #4 <java/lang/System.out>
35 aload_2
36 invokevirtual #6 <java/io/PrintStream.println>
39 return
当对现有的字符串进行连接操作时,也需要重新指定内存区域赋值,不能使用原有的value进行赋值
@Test
public void test2() {
String s1 = "abc";
String s2 = "abc";
s2 += "def";
System.out.println(s2);//abcdef
System.out.println(s1);//abc
}
0 ldc #2 <abc>
2 astore_1
3 ldc #2 <abc>
5 astore_2
6 new #7 <java/lang/StringBuilder>
9 dup
10 invokespecial #8 <java/lang/StringBuilder.<init>>
13 aload_2
14 invokevirtual #9 <java/lang/StringBuilder.append>
17 ldc #10 <def>
19 invokevirtual #9 <java/lang/StringBuilder.append>
22 invokevirtual #11 <java/lang/StringBuilder.toString>
25 astore_2
26 getstatic #4 <java/lang/System.out>
29 aload_2
30 invokevirtual #6 <java/io/PrintStream.println>
33 getstatic #4 <java/lang/System.out>
36 aload_1
37 invokevirtual #6 <java/io/PrintStream.println>
40 return
当调用string的replace()方法修改指定字符或字符串时,也需要重新指定内存区域赋值,不能使用原有的value进行赋值
@Test
public void test3() {
String s1 = "abc";
String s2 = s1.replace('a', 'm');
System.out.println(s1);//abc
System.out.println(s2);//mbc
}
来看看 replace() 方法的源码
public String replace(char oldChar, char newChar) {
if (oldChar != newChar) {
int len = value.length;
int i = -1;
char[] val = value; /* avoid getfield opcode */
while (++i < len) {
if (val[i] == oldChar) {
break;
}
}
if (i < len) {
char buf[] = new char[len];
for (int j = 0; j < i; j++) {
buf[j] = val[j];
}
while (i < len) {
char c = val[i];
buf[i] = (c == oldChar) ? newChar : c;
i++;
}
return new String(buf, true);
}
}
return this;
}
课后练习:String 的不可变性
/** * @author shkstart shkstart@126.com * @create 2020 23:44 */public class StringExer { String str = new String("good"); char[] ch = {'t', 'e', 's', 't'}; public void change(String str, char ch[]) { str = "test ok"; ch[0] = 'b'; } public static void main(String[] args) { StringExer ex = new StringExer(); ex.change(ex.str, ex.ch); System.out.println(ex.str);//good System.out.println(ex.ch);//best }}
goodbest
String 底层 Hashtable 结构的说明
字符串常量池是不会存储相同内容的字符串的
代码示例:设置 StringTable 的长度
/** * -XX:StringTableSize=1009 * * @author shkstart shkstart@126.com * @create 2020 23:53 */public class StringTest2 { public static void main(String[] args) { // 测试StringTableSize参数 System.out.println("我来打个酱油"); try { Thread.sleep(1000000); } catch (InterruptedException e) { e.printStackTrace(); } }}1234567891011121314151617
通过 -XX:StringTableSize 设置 StringTable 长度
-XX:StringTableSize=6666
jpsjinfo -flag StringTableSize 进程id
测试不同 StringTable 长度下,程序的性能
/** * -XX:StringTableSize=1009 * * @author shkstart shkstart@126.com * @create 2020 23:53 */public class StringTest2 { public static void main(String[] args) { BufferedReader br = null; try { br = new BufferedReader(new FileReader("words.txt")); long start = System.currentTimeMillis(); String data; while ((data = br.readLine()) != null) { //如果字符串常量池中没有对应data的字符串的话,则在常量池中生成 data.intern(); } long end = System.currentTimeMillis(); System.out.println("花费的时间为:" + (end - start));//1009:143ms 100009:47ms } catch (IOException e) { e.printStackTrace(); } finally { if (br != null) { try { br.close(); } catch (IOException e) { e.printStackTrace(); } } } }}
String 类型
String info="atguigu.com";
String 内存分配的演进过程
StringTable 为什么要调整?
代码示例
/** * jdk6中: * -XX:PermSize=6m -XX:MaxPermSize=6m -Xms6m -Xmx6m * * jdk8中: * -XX:MetaspaceSize=6m -XX:MaxMetaspaceSize=6m -Xms6m -Xmx6m * @author shkstart shkstart@126.com * @create 2020 0:36 */public class StringTest3 { public static void main(String[] args) { //使用Set保持着常量池引用,避免full gc回收常量池行为 Set<String> set = new HashSet<String>(); //在short可以取值的范围内足以让6MB的PermSize或heap产生OOM了。 short i = 0; while(true){ set.add(String.valueOf(i++).intern()); } }}
"C:\Program Files\Java\jdk1.8.0_144\bin\java" -XX:MetaspaceSize=6m -XX:MaxMetaspaceSize=6m -Xms6m -Xmx6m "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2017.3.1\lib\idea_rt.jar=1799:C:\Program Files\JetBrains\IntelliJ IDEA 2017.3.1\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_144\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\rt.jar;C:\Users\Heygo\Desktop\JVMDemo\out\production\chapter13;D:\JavaTools\apache-maven-3.3.9\repository\junit\junit\4.12\junit-4.12.jar;D:\JavaTools\apache-maven-3.3.9\repository\org\hamcrest\hamcrest-core\1.3\hamcrest-core-1.3.jar" com.atguigu.java.StringTest3Exception in thread "main" java.lang.OutOfMemoryError: Java heap space at java.util.HashMap.resize(HashMap.java:703) at java.util.HashMap.putVal(HashMap.java:662) at java.util.HashMap.put(HashMap.java:611) at java.util.HashSet.add(HashSet.java:219) at com.atguigu.java.StringTest3.main(StringTest3.java:22)Process finished with exit code 1
核心思想
Java语言规范里要求完全相同的字符串字面量,应该包含同样的Unicode字符序列(包含同一份码点序列的常量),并且必须是指向同一个String类实例。
题目一
/** * @author shkstart shkstart@126.com * @create 2020 0:49 */public class StringTest4 { public static void main(String[] args) { System.out.println();//2330 System.out.println("1");//2331 System.out.println("2"); System.out.println("3"); System.out.println("4"); System.out.println("5"); System.out.println("6"); System.out.println("7"); System.out.println("8"); System.out.println("9"); System.out.println("10");//2340 //如下的字符串"1" 到 "10"不会再次加载 System.out.println("1");//2341 System.out.println("2");//2341 System.out.println("3"); System.out.println("4"); System.out.println("5"); System.out.println("6"); System.out.println("7"); System.out.println("8"); System.out.println("9"); System.out.println("10");//2341 }}
题目二
/** * @author shkstart shkstart@126.com * @create 2020 0:51 */class Memory { public static void main(String[] args) {//line 1 int i = 1;//line 2 Object obj = new Object();//line 3 Memory mem = new Memory();//line 4 mem.foo(obj);//line 5 }//line 9 private void foo(Object param) {//line 6 String str = param.toString();//line 7 System.out.println(str); }//line 8}
字符串拼接操作的结论
常量与常量的拼接结果在常量池,原理是编译期优化
@Testpublic void test1() { String s1 = "a" + "b" + "c";//编译期优化:等同于"abc" String s2 = "abc"; //"abc"一定是放在字符串常量池中,将此地址赋给s2 /* * 最终.java编译成.class,再执行.class * String s1 = "abc"; * String s2 = "abc" */ System.out.println(s1 == s2); //true System.out.println(s1.equals(s2)); //true}
0 ldc #2 <abc> 2 astore_1 3 ldc #2 <abc> 5 astore_2 6 getstatic #3 <java/lang/System.out> 9 aload_110 aload_211 if_acmpne 18 (+7)14 iconst_115 goto 19 (+4)18 iconst_019 invokevirtual #4 <java/io/PrintStream.println>22 getstatic #3 <java/lang/System.out>25 aload_126 aload_227 invokevirtual #5 <java/lang/String.equals>30 invokevirtual #4 <java/io/PrintStream.println>33 return
拼接前后,只要其中有一个是变量,结果就在堆中
调用 intern() 方法,则主动将字符串对象存入字符串常量池中,并将其地址返回
@Testpublic void test2(){ String s1 = "javaEE"; String s2 = "hadoop"; String s3 = "javaEEhadoop"; String s4 = "javaEE" + "hadoop";//编译期优化 //如果拼接符号的前后出现了变量,则相当于在堆空间中new String(),具体的内容为拼接的结果:javaEEhadoop String s5 = s1 + "hadoop"; String s6 = "javaEE" + s2; String s7 = s1 + s2; System.out.println(s3 == s4);//true System.out.println(s3 == s5);//false System.out.println(s3 == s6);//false System.out.println(s3 == s7);//false System.out.println(s5 == s6);//false System.out.println(s5 == s7);//false System.out.println(s6 == s7);//false //intern():判断字符串常量池中是否存在javaEEhadoop值,如果存在,则返回常量池中javaEEhadoop的地址; //如果字符串常量池中不存在javaEEhadoop,则在常量池中加载一份javaEEhadoop,并返回此对象的地址。 String s8 = s6.intern(); System.out.println(s3 == s8);//true}
0 ldc #6 <javaEE> 2 astore_1 3 ldc #7 <hadoop> 5 astore_2 6 ldc #8 <javaEEhadoop> 8 astore_3 9 ldc #8 <javaEEhadoop> 11 astore 4 13 new #9 <java/lang/StringBuilder> 16 dup 17 invokespecial #10 <java/lang/StringBuilder.<init>> 20 aload_1 21 invokevirtual #11 <java/lang/StringBuilder.append> 24 ldc #7 <hadoop> 26 invokevirtual #11 <java/lang/StringBuilder.append> 29 invokevirtual #12 <java/lang/StringBuilder.toString> 32 astore 5 34 new #9 <java/lang/StringBuilder> 37 dup 38 invokespecial #10 <java/lang/StringBuilder.<init>> 41 ldc #6 <javaEE> 43 invokevirtual #11 <java/lang/StringBuilder.append> 46 aload_2 47 invokevirtual #11 <java/lang/StringBuilder.append> 50 invokevirtual #12 <java/lang/StringBuilder.toString> 53 astore 6 55 new #9 <java/lang/StringBuilder> 58 dup 59 invokespecial #10 <java/lang/StringBuilder.<init>> 62 aload_1 63 invokevirtual #11 <java/lang/StringBuilder.append> 66 aload_2 67 invokevirtual #11 <java/lang/StringBuilder.append> 70 invokevirtual #12 <java/lang/StringBuilder.toString> 73 astore 7 75 getstatic #3 <java/lang/System.out> 78 aload_3 79 aload 4 81 if_acmpne 88 (+7) 84 iconst_1 85 goto 89 (+4) 88 iconst_0 89 invokevirtual #4 <java/io/PrintStream.println> 92 getstatic #3 <java/lang/System.out> 95 aload_3 96 aload 5 98 if_acmpne 105 (+7)101 iconst_1102 goto 106 (+4)105 iconst_0106 invokevirtual #4 <java/io/PrintStream.println>109 getstatic #3 <java/lang/System.out>112 aload_3113 aload 6115 if_acmpne 122 (+7)118 iconst_1119 goto 123 (+4)122 iconst_0123 invokevirtual #4 <java/io/PrintStream.println>126 getstatic #3 <java/lang/System.out>129 aload_3130 aload 7132 if_acmpne 139 (+7)135 iconst_1136 goto 140 (+4)139 iconst_0140 invokevirtual #4 <java/io/PrintStream.println>143 getstatic #3 <java/lang/System.out>146 aload 5148 aload 6150 if_acmpne 157 (+7)153 iconst_1154 goto 158 (+4)157 iconst_0158 invokevirtual #4 <java/io/PrintStream.println>161 getstatic #3 <java/lang/System.out>164 aload 5166 aload 7168 if_acmpne 175 (+7)171 iconst_1172 goto 176 (+4)175 iconst_0176 invokevirtual #4 <java/io/PrintStream.println>179 getstatic #3 <java/lang/System.out>182 aload 6184 aload 7186 if_acmpne 193 (+7)189 iconst_1190 goto 194 (+4)193 iconst_0194 invokevirtual #4 <java/io/PrintStream.println>197 aload 6199 invokevirtual #13 <java/lang/String.intern>202 astore 8204 getstatic #3 <java/lang/System.out>207 aload_3208 aload 8210 if_acmpne 217 (+7)213 iconst_1214 goto 218 (+4)217 iconst_0218 invokevirtual #4 <java/io/PrintStream.println>221 return
字符串拼接的底层细节
代码示例 1
@Testpublic void test3(){ String s1 = "a"; String s2 = "b"; String s3 = "ab"; /* 如下的s1 + s2 的执行细节:(变量s是我临时定义的) ① StringBuilder s = new StringBuilder(); ② s.append("a") ③ s.append("b") ④ s.toString() --> 约等于 new String("ab") 补充:在jdk5.0之后使用的是StringBuilder,在jdk5.0之前使用的是StringBuffer */ String s4 = s1 + s2;//"ab" System.out.println(s3 == s4);//false}
0 ldc #14 <a> 2 astore_1 3 ldc #15 <b> 5 astore_2 6 ldc #16 <ab> 8 astore_3 9 new #9 <java/lang/StringBuilder>12 dup13 invokespecial #10 <java/lang/StringBuilder.<init>>16 aload_117 invokevirtual #11 <java/lang/StringBuilder.append>20 aload_221 invokevirtual #11 <java/lang/StringBuilder.append>24 invokevirtual #12 <java/lang/StringBuilder.toString>27 astore 429 getstatic #3 <java/lang/System.out>32 aload_333 aload 435 if_acmpne 42 (+7)38 iconst_139 goto 43 (+4)42 iconst_043 invokevirtual #4 <java/io/PrintStream.println>46 return
代码示例 2
/*1. 字符串拼接操作不一定使用的是StringBuilder! 如果拼接符号左右两边都是字符串常量或常量引用,则仍然使用编译期优化,即非StringBuilder的方式。2. 针对于final修饰类、方法、基本数据类型、引用数据类型的量的结构时,能使用上final的时候建议使用上。 */@Testpublic void test4(){ final String s1 = "a"; final String s2 = "b"; String s3 = "ab"; String s4 = s1 + s2; System.out.println(s3 == s4);//true}
0 ldc #14 <a> 2 astore_1 3 ldc #15 <b> 5 astore_2 6 ldc #16 <ab> 8 astore_3 9 ldc #16 <ab>11 astore 413 getstatic #3 <java/lang/System.out>16 aload_317 aload 419 if_acmpne 26 (+7)22 iconst_123 goto 27 (+4)26 iconst_027 invokevirtual #4 <java/io/PrintStream.println>30 return
课后练习
//练习:@Testpublic void test5(){ String s1 = "javaEEhadoop"; String s2 = "javaEE"; String s3 = s2 + "hadoop"; System.out.println(s1 == s3);//false final String s4 = "javaEE";//s4:常量 String s5 = s4 + "hadoop"; System.out.println(s1 == s5);//true}
ldc #8 <javaEEhadoop>
(带 final 的变量在编译时就已经确定了该变量的值,当做常量来处理) 0 ldc #8 <javaEEhadoop> 2 astore_1 3 ldc #6 <javaEE> 5 astore_2 6 new #9 <java/lang/StringBuilder> 9 dup10 invokespecial #10 <java/lang/StringBuilder.<init>>13 aload_214 invokevirtual #11 <java/lang/StringBuilder.append>17 ldc #7 <hadoop>19 invokevirtual #11 <java/lang/StringBuilder.append>22 invokevirtual #12 <java/lang/StringBuilder.toString>25 astore_326 getstatic #3 <java/lang/System.out>29 aload_130 aload_331 if_acmpne 38 (+7)34 iconst_135 goto 39 (+4)38 iconst_039 invokevirtual #4 <java/io/PrintStream.println>42 ldc #6 <javaEE>44 astore 446 ldc #8 <javaEEhadoop>48 astore 550 getstatic #3 <java/lang/System.out>53 aload_154 aload 556 if_acmpne 63 (+7)59 iconst_160 goto 64 (+4)63 iconst_064 invokevirtual #4 <java/io/PrintStream.println>67 return
拼接操作与 append 操作的效率对比
/*体会执行效率:通过StringBuilder的append()的方式添加字符串的效率要远高于使用String的字符串拼接方式!分析原因: ① StringBuilder的append()的方式: 自始至终中只创建过一个StringBuilder的对象 使用String的字符串拼接方式:创建过多个StringBuilder和String的对象 ② 使用String的字符串拼接方式: 内存中由于创建了较多的StringBuilder和String的对象,内存占用更大; 如果进行GC,需要花费额外的时间。 改进的空间: 在实际开发中,如果基本确定要前前后后添加的字符串长度不高于某个限定值highLevel的情况下,建议使用构造器实例化: StringBuilder s = new StringBuilder(highLevel);//new char[highLevel] */@Testpublic void test6(){ long start = System.currentTimeMillis(); // method1(100000);//4014 method2(100000);//7 long end = System.currentTimeMillis(); System.out.println("花费的时间为:" + (end - start));}public void method1(int highLevel){ String src = ""; for(int i = 0;i < highLevel;i++){ src = src + "a";//每次循环都会创建一个StringBuilder、String }}public void method2(int highLevel){ //只需要创建一个StringBuilder StringBuilder src = new StringBuilder(); for (int i = 0; i < highLevel; i++) { src.append("a"); }}
StringBuilder s = new StringBuilder(highLevel); //new char[highLevel]
通过字节码分析
0 ldc #23 2 astore_2 3 iconst_0 4 istore_3 5 iload_3 6 iload_1 7 if_icmpge 36 (+29)10 new #9 <java/lang/StringBuilder>13 dup14 invokespecial #10 <java/lang/StringBuilder.<init>>17 aload_218 invokevirtual #11 <java/lang/StringBuilder.append>21 ldc #14 <a>23 invokevirtual #11 <java/lang/StringBuilder.append>26 invokevirtual #12 <java/lang/StringBuilder.toString>29 astore_230 iinc 3 by 133 goto 5 (-28)36 return
0 new #9 <java/lang/StringBuilder> 3 dup 4 invokespecial #10 <java/lang/StringBuilder.<init>> 7 astore_2 8 iconst_0 9 istore_310 iload_311 iload_112 if_icmpge 28 (+16)15 aload_216 ldc #14 <a>18 invokevirtual #11 <java/lang/StringBuilder.append>21 pop22 iinc 3 by 125 goto 10 (-15)28 return
关于 StringBuilder 构造器
public StringBuilder(int capacity) { super(capacity);}
AbstractStringBuilder(int capacity) { value = new char[capacity];}
intern() 方法的说明
先来点逼格,看看官方文档
When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.
It follows that for any two strings s and t, s.intern() == t.intern() is true if and only if s.equals(t) is true.
public native String intern();
关于 intern() 方法的说明
new String(“ab”)会创建几个对象?
/** * 题目: * new String("ab")会创建几个对象?看字节码,就知道是两个。 * 一个对象是:new关键字在堆空间创建的 * 另一个对象是:字符串常量池中的对象"ab"。 字节码指令:ldc * * @author shkstart shkstart@126.com * @create 2020 20:38 */public class StringNewTest { public static void main(String[] args) { String str = new String("ab"); }}
0 new #2 <java/lang/String> 3 dup 4 ldc #3 <ab> 6 invokespecial #4 <java/lang/String.<init>> 9 astore_110 return
0 new #2 <java/lang/String>
:在堆中创建了一个 String 对象4 ldc #3 <ab>
:在字符串常量池中放入 “ab”(如果之前字符串常量池中没有 “ab” 的话)new String(“a”) + new String(“b”) 会创建几个对象?
/** * 思考: * new String("a") + new String("b")呢? * 对象1:new StringBuilder() * 对象2: new String("a") * 对象3: 常量池中的"a" * 对象4: new String("b") * 对象5: 常量池中的"b" * * 深入剖析: StringBuilder的toString(): * 对象6 :new String("ab") * 强调一下,toString()的调用,在字符串常量池中,没有生成"ab" * * @author shkstart shkstart@126.com * @create 2020 20:38 */public class StringNewTest { public static void main(String[] args) { String str = new String("a") + new String("b"); }}
0 new #2 <java/lang/StringBuilder> 3 dup 4 invokespecial #3 <java/lang/StringBuilder.<init>> 7 new #4 <java/lang/String>10 dup11 ldc #5 <a>13 invokespecial #6 <java/lang/String.<init>>16 invokevirtual #7 <java/lang/StringBuilder.append>19 new #4 <java/lang/String>22 dup23 ldc #8 <b>25 invokespecial #6 <java/lang/String.<init>>28 invokevirtual #7 <java/lang/StringBuilder.append>31 invokevirtual #9 <java/lang/StringBuilder.toString>34 astore_135 return
0 new #2 <java/lang/StringBuilder>
:拼接字符串会创建一个 StringBuilder 对象7 new #4 <java/lang/String>
:创建 String 对象,对应于 new String(“a”)11 ldc #5 <a>
:在字符串常量池中放入 “a”(如果之前字符串常量池中没有 “a” 的话)19 new #4 <java/lang/String>
:创建 String 对象,对应于 new String(“b”)23 ldc #8 <b>
:在字符串常量池中放入 “b”(如果之前字符串常量池中没有 “b” 的话)31 invokevirtual #9 <java/lang/StringBuilder.toString>
:调用 StringBuilder 的 toString() 方法,会生成一个 String 对象深入剖析 StringBuilder 的toString() 方法
@Overridepublic String toString() { // Create a copy, don't share the array return new String(value, 0, count);}
char[] value;
有点难的面试题
/** * 如何保证变量s指向的是字符串常量池中的数据呢?有两种方式: * 方式一: String s = "shkstart";//字面量定义的方式 * 方式二: 调用intern() * String s = new String("shkstart").intern(); * String s = new StringBuilder("shkstart").toString().intern(); * * @author shkstart shkstart@126.com * @create 2020 18:49 */public class StringIntern { public static void main(String[] args) { String s = new String("1"); s.intern();//这方法其实没啥屌用,调用此方法之前,字符串常量池中已经存在"1" String s2 = "1"; /* jdk6:false jdk7/8:false 因为 s 指向堆空间中的 "1" ,s2 指向字符创常量池中的 "1" */ System.out.println(s == s2); // 执行完下一行代码以后,字符串常量池中,是否存在"11"呢?答案:不存在!! String s3 = new String("1") + new String("1");//s3变量记录的地址为:new String("11") /* 如何理解:jdk6:创建了一个新的对象"11",也就有新的地址。 jdk7:此时常量中并没有创建"11",而是在常量池中记录了指向堆空间中new String("11")的地址(节省空间) */ s3.intern(); // 在字符串常量池中生成"11"。 String s4 = "11";//s4变量记录的地址:使用的是上一行代码代码执行时,在常量池中生成的"11"的地址 // jdk6:false jdk7/8:true System.out.println(s3 == s4); }}
内存分析
面试题的拓展
/** * StringIntern.java中练习的拓展: * * @author shkstart shkstart@126.com * @create 2020 22:10 */public class StringIntern1 { public static void main(String[] args) { //执行完下一行代码以后,字符串常量池中,是否存在"11"呢?答案:不存在!! String s3 = new String("1") + new String("1");//new String("11") //在字符串常量池中生成对象"11" String s4 = "11"; String s5 = s3.intern(); // s3 是堆中的 "ab" ,s4 是字符串常量池中的 "ab" System.out.println(s3 == s4);//false // s5 是从字符串常量池中取回来的引用,当然和 s4 相等 System.out.println(s5 == s4);//true }}
关于 intern() 的总结
intern() 方法的课后练习
练习 1
/** * @author shkstart shkstart@126.com * @create 2020 20:17 */public class StringExer1 { public static void main(String[] args) { //在下一行代码执行完以后,字符串常量池中并没有"ab" String s = new String("a") + new String("b");//new String("ab") /* jdk6中:在串池中创建一个字符串"ab" jdk8中:串池中没有创建字符串"ab",而是创建一个引用,指向new String("ab"),将此引用返回 */ String s2 = s.intern(); System.out.println(s2 == "ab");//jdk6:true jdk8:true System.out.println(s == "ab");//jdk6:false jdk8:true }}
练习 2
/** * @author shkstart shkstart@126.com * @create 2020 20:17 */public class StringExer1 { public static void main(String[] args) { // 在这儿加一句 String x = "ab"; //在下一行代码执行完以后,字符串常量池中并没有"ab" String s = new String("a") + new String("b");//new String("ab") /* jdk6中:在串池中创建一个字符串"ab" jdk8中:串池中没有创建字符串"ab",而是创建一个引用,指向new String("ab"),将此引用返回 */ String s2 = s.intern(); System.out.println(s2 == "ab");//jdk6:true jdk8:true System.out.println(s == "ab");//jdk6:false jdk8:false }}
练习 3
/** * * @author shkstart shkstart@126.com * @create 2020 20:26 */public class StringExer2 { public static void main(String[] args) { String s1 = new String("ab");//执行完以后,会在字符串常量池中会生成"ab" s1.intern(); String s2 = "ab"; System.out.println(s1 == s2); // false }}
/** * * @author shkstart shkstart@126.com * @create 2020 20:26 */public class StringExer2 { // 对象内存地址可以使用System.identityHashCode(object)方法获取 public static void main(String[] args) { String s1 = new String("a") + new String("b");//执行完以后,不会在字符串常量池中会生成"ab" System.out.println(System.identityHashCode(s1)); s1.intern(); System.out.println(System.identityHashCode(s1)); String s2 = "ab"; System.out.println(System.identityHashCode(s2)); System.out.println(s1 == s2); // true }}/* 程序运行结果 21685669 21685669 21685669 true*/
intern() 的效率测试
/** * 使用intern()测试执行效率:空间使用上 * 结论:对于程序中大量存在存在的字符串,尤其其中存在很多重复字符串时,使用intern()可以节省内存空间。 * * @author shkstart shkstart@126.com * @create 2020 21:17 */public class StringIntern2 { static final int MAX_COUNT = 1000 * 10000; static final String[] arr = new String[MAX_COUNT]; public static void main(String[] args) { Integer[] data = new Integer[]{1,2,3,4,5,6,7,8,9,10}; long start = System.currentTimeMillis(); for (int i = 0; i < MAX_COUNT; i++) { // arr[i] = new String(String.valueOf(data[i % data.length])); arr[i] = new String(String.valueOf(data[i % data.length])).intern(); } long end = System.currentTimeMillis(); System.out.println("花费的时间为:" + (end - start)); try { Thread.sleep(1000000); } catch (InterruptedException e) { e.printStackTrace(); } System.gc(); }}
结论:
/** * String的垃圾回收: * -Xms15m -Xmx15m -XX:+PrintStringTableStatistics -XX:+PrintGCDetails * * @author shkstart shkstart@126.com * @create 2020 21:27 */public class StringGCTest { public static void main(String[] args) { for (int j = 0; j < 100000; j++) { String.valueOf(j).intern(); } }}
-Xms15m -Xmx15m -XX:+PrintStringTableStatistics -XX:+PrintGCDetails
"C:\Program Files\Java\jdk1.8.0_144\bin\java" -Xms15m -Xmx15m -XX:+PrintStringTableStatistics -XX:+PrintGCDetails "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2017.3.1\lib\idea_rt.jar=11487:C:\Program Files\JetBrains\IntelliJ IDEA 2017.3.1\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_144\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\rt.jar;C:\Users\Heygo\Desktop\JVMDemo\out\production\chapter13;D:\JavaTools\apache-maven-3.3.9\repository\junit\junit\4.12\junit-4.12.jar;D:\JavaTools\apache-maven-3.3.9\repository\org\hamcrest\hamcrest-core\1.3\hamcrest-core-1.3.jar" com.atguigu.java3.StringGCTest[GC (Allocation Failure) [PSYoungGen: 4096K->488K(4608K)] 4096K->716K(15872K), 0.0024275 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] Heap PSYoungGen total 4608K, used 3883K [0x00000000ffb00000, 0x0000000100000000, 0x0000000100000000) eden space 4096K, 82% used [0x00000000ffb00000,0x00000000ffe50fb0,0x00000000fff00000) from space 512K, 95% used [0x00000000fff00000,0x00000000fff7a020,0x00000000fff80000) to space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000) ParOldGen total 11264K, used 228K [0x00000000ff000000, 0x00000000ffb00000, 0x00000000ffb00000) object space 11264K, 2% used [0x00000000ff000000,0x00000000ff039010,0x00000000ffb00000) Metaspace used 3472K, capacity 4496K, committed 4864K, reserved 1056768K class space used 381K, capacity 388K, committed 512K, reserved 1048576KSymbolTable statistics:Number of buckets : 20011 = 160088 bytes, avg 8.000Number of entries : 14158 = 339792 bytes, avg 24.000Number of literals : 14158 = 603200 bytes, avg 42.605Total footprint : = 1103080 bytesAverage bucket size : 0.708Variance of bucket size : 0.711Std. dev. of bucket size: 0.843Maximum bucket size : 6StringTable statistics:Number of buckets : 60013 = 480104 bytes, avg 8.000Number of entries : 62943 = 1510632 bytes, avg 24.000Number of literals : 62943 = 3584040 bytes, avg 56.941Total footprint : = 5574776 bytesAverage bucket size : 1.049Variance of bucket size : 0.824Std. dev. of bucket size: 0.908Maximum bucket size : 5Process finished with exit code 0
String.valueOf() 方法源码
public static String valueOf(int i) { return Integer.toString(i);}
public static String toString(int i) { if (i == Integer.MIN_VALUE) return "-2147483648"; int size = (i < 0) ? stringSize(-i) + 1 : stringSize(i); char[] buf = new char[size]; getChars(i, size, buf); return new String(buf, true);}
官方文档
http://openjdk.java.net/jeps/192
String 去重操作的背景
String 去重的的具体实现
命令行选项