干货总结｜快速构造String对象及访问其内部成员的技巧
阿
温绍锦(高铁)约 8 分钟
温绍锦(高铁)约 8 分钟
一、相关知识JDK String的实现
字符串在JDK 8及JDK 9之后的实现有很大不同的。JDK 8中，String的结构是这样的：class String {char[] value;// 构造函数会拷贝public String(char value[]) {this.value = Arrays.copyOf(value, value.length);}// 无拷贝构造函数String(char[] value, boolean share) {// assert share : "unshared not supported";this.value = value;}}
class String {static final byte LATIN1 = 0;static final byte UTF16 = 1;byte code;byte[] value;// 无拷贝构造函数String(byte[] value, byte coder) {this.value = value;this.coder = coder;}}
二、相关知识Unsafe
JDK 8之后提供sun.Unsafe可以做一些原生的操作，性能更好，不安全，错误的调用会导致JVM Crash。如果用对了，能提升性能。Unsafe能帮你绕过任何限制。public class UnsafeUtils {public static final Unsafe UNSAFE;static {Unsafe unsafe = null;try {Field theUnsafeField = Unsafe.class.getDeclaredField("theUnsafe");theUnsafeField.setAccessible(true);unsafe = (Unsafe) theUnsafeField.get(null);} catch (Throwable ignored) {// ignored}UNSAFE = unsafe;}}
三、相关知识Trusted MethodHandles.Lookup
JDK 8开始支持Lambda，为了方便将一个Method映射为一个Lambda Function，避免反射开销。java.invoke.LambdaMetafactory可以实现这一功能，但这个也受限于可见性的限制，也就是说不能调用私有方法。有一个技巧，结合Unsafe，可以在不同版本的JDK都能构造一个Trusted MethodHandles.Lookup来绕开可见性的限制，调用任何JDK内部方法。如下：import static com.alibaba.fastjson2.util.UnsafeUtils.UNSAFE;static final MethodHandles.Lookup IMPL_LOOKUP;static {Class lookupClass = MethodHandles.Lookup.class;Field implLookup = lookupClass.getDeclaredField("IMPL_LOOKUP");long fieldOffset = UNSAFE.staticFieldOffset(implLookup);IMPL_LOOKUP = (MethodHandles.Lookup) UNSAFE.getObject(lookupClass, fieldOffset);}static MethodHandles.Lookup trustedLookup(Class objectClass) throws Exception {return IMPL_LOOKUP.in(objectClass);}
四、零拷贝构造String对象
快速构造字符串的关键是要做减少拷贝，甚至零拷贝，在JDK 8、JDK 9~15、JDK 16及之后的版本的实现都不一样。由于String(char[], boolean)方法不是public的，上面的代码会报错，要通过反射构造一个TRUSTED的MethodHandles.Lookup，然调用String的内部方法，映射成一个BiFunction<char[], Boolean, String>，代码如下：BiFunction<char[], Boolean, String> stringCreatorJDK8= (char[] value, boolean share) -> new String(chars, boolean);
import com.alibaba.fastjson2.util.JDKUtils;import java.util.function.BiFunction;import java.lang.invoke.MethodHandles;import static java.lang.invoke.MethodType.methodType;MethodHandles.Lookup caller = JDKUtils.trustedLookup(String.class);MethodHandle handle = caller.findConstructor(String.class,methodType(void.class, char[].class, boolean.class));CallSite callSite = LambdaMetafactory.metafactory(caller,"apply",methodType(BiFunction.class),methodType(Object.class, Object.class, Object.class),handle,methodType(String.class, char[].class, boolean.class));BiFunction<char[], Boolean, String> STRING_CREATOR_JDK8= (BiFunction<char[], Boolean, String>)callSite.getTarget().invokeExact();
同样，JDK 9中的String(byte[], byte)方法不是public，无法直接调用，上面的代码会报错，要构造一个TRUSTED MethodHandles.Lookup方法调用String内部方法，如下：BiFunction<byte[], Byte, String> STRING_CREATOR_JDK11= (byte[] value, byte coder) -> new String(value, coder);
注意：当用户配置JVM参数-XX:-CompactStrings时，上述方法无效。import com.alibaba.fastjson2.util.JDKUtils;import static java.lang.invoke.MethodType.methodType;MethodHandles.Lookup caller = JDKUtils.trustedLookup(String.class);MethodHandle handle = caller.findConstructor(String.class,methodType(void.class, byte[].class, byte.class));CallSite callSite = LambdaMetafactory.metafactory(caller,"apply",methodType(BiFunction.class),methodType(Object.class, Object.class, Object.class),handle,methodType(String.class, byte[].class, Byte.class));BiFunction<byte[], Byte, String> STRING_CREATOR_JDK11= (BiFunction<byte[], Byte, String>)callSite.getTarget().invokeExact();
stiatic BiFunction<char[], Boolean, String> STRING_CREATOR_JDK8 = ...static BiFunction<byte[], Byte, String> STRING_CREATOR_JDK11 = ...static String formatYYYYMMDD(LocalDate date) {int year = date.getYear();int month = date.getMonthValue();int dayOfMonth = date.getDayOfMonth();int y0 = year / 1000 + '0';int y1 = (year / 100) % 10 + '0';int y2 = (year / 10) % 10 + '0';int y3 = year % 10 + '0';int m0 = month / 10 + '0';int m1 = month % 10 + '0';int d0 = dayOfMonth / 10 + '0';int d1 = dayOfMonth % 10 + '0';String str;if (STRING_CREATOR_JDK11 != null) {byte[] bytes = new byte[10];bytes[0] = (byte) y0;bytes[1] = (byte) y1;bytes[2] = (byte) y2;bytes[3] = (byte) y3;bytes[4] = '-';bytes[5] = (byte) m0;bytes[6] = (byte) m1;bytes[7] = '-';bytes[8] = (byte) d0;bytes[9] = (byte) d1;str = STRING_CREATOR_JDK11.apply(bytes, JDKUtils.LATIN1);} else {char[] chars = new char[10];chars[0] = (char) y1;chars[1] = (char) y2;chars[2] = (char) y3;chars[3] = (char) y4;chars[4] = '-';chars[5] = (char) m0;chars[6] = (char) m1;chars[7] = '-';chars[8] = (char) d0;chars[9] = (char) d1;if (STRING_CREATOR_JDK8 != null) {str = STRING_CREATOR_JDK8.apply(chars, Boolean.TRUE);} else {str = new String(chars);}}return str;}
五、直接访问String对象内部成员
static final Field FIELD_STRING_VALUE;static final long FIELD_STRING_VALUE_OFFSET;static {Field field = null;long fieldOffset = -1;try {field = String.class.getDeclaredField("value");fieldOffset = UnsafeUtils.objectFieldOffset(field);} catch (Exception ignored) {FIELD_STRING_ERROR = true;}FIELD_STRING_VALUE = field;FIELD_STRING_VALUE_OFFSET = fieldOffset;}public static char[] getCharArray(String str) {if (!FIELD_STRING_ERROR) {try {return (char[]) UnsafeUtils.UNSAFE.getObject(str,FIELD_STRING_VALUE_OFFSET);} catch (Exception ignored) {FIELD_STRING_ERROR = true;}}return str.toCharArray();}
但由于String.coder和value方法不是public可见的，和上面的4.2类似，要通过TRUSTED MethodHandles.Lookup构造，如下：ToIntFunction<String> stringCoder = (String str) -> str.coder();Function<String, byte[]> stringValue = (String str) -> str.value();
import com.alibaba.fastjson2.util.JDKUtils;import static java.lang.invoke.MethodType.methodType;MethodHandles.Lookup lookup = JDKUtils.trustedLookup(String.class);MethodHandle coder = lookup.findSpecial(String.class,"coder",methodType(byte.class),String.class);CallSite applyAsInt = LambdaMetafactory.metafactory(lookup,"applyAsInt",methodType(ToIntFunction.class),methodType(int.class, Object.class),coder,MethodType.methodType(byte.class, String.class));ToIntFunction<String> STRING_CODER= (ToIntFunction<String>) applyAsInt.getTarget().invokeExact();MethodHandle value = lookup.findSpecial(String.class,"value",methodType(byte[].class),String.class);CallSite apply = LambdaMetafactory.metafactory(lookup,"apply",methodType(Function.class),methodType(Object.class, Object.class),value,methodType(byte[].class, String.class));Function<String, byte[]> STRING_VALUE= (Function<String, byte[]>) apply.getTarget().invokeExact();
static Byte LATIN1 = 0;static ToIntFunction<String> STRING_CODER = ...static Function<String, byte[]> STRING_VALUE ...byte[] buf = ...;int off;void writeString(string str) {if (STRING_CODER != null && STRING_VALUE != null) {// improved for JDK 9 LATIN1int coder = stringCoder.apply(str);if (coder == LATIN1) {// str.getBytes(0, str.length, buf, off);byte[] value = STRING_VALUE.apply(str);System.arrayCopy(value, 0, buf, off, value.length);return;}}// normal logic}
class String {@Deprecatedpublic void getBytes(int srcBegin, int srcEnd, byte dst[], int dstBegin) {int j = dstBegin;int n = srcEnd;int i = srcBegin;char[] val = value; /* avoid getfield opcode */while (i < n) {dst[j++] = (byte)val[i++];}}}static Byte LATIN1 = 0;static ToIntFunction<String> STRING_CODER = ...byte[] buf = ...;int off;void writeString(string str) {if (STRING_CODER != null) {// improved for JDK 9 LATIN1int coder = STRING_CODER.apply(str);if (coder == LATIN1) {str.getBytes(0, str.length, buf, off);return;}}// normal logic}
参考实现：
FASTJSON2项目使用了上面的技巧，其中JDKUtils和UnsafeUtils有上面技巧的实现：-
JDKUtils： https://github.com/alibaba/fastjson2/blob/fastcode_demo_20221218/core/src/main/java/com/alibaba/fastjson2/util/JDKUtils.java -
UnsafeUtils： https://github.com/alibaba/fastjson2/blob/fastcode_demo_20221218/core/src/main/java/com/alibaba/fastjson2/util/UnsafeUtils.java