在后面的文章《一探到底-如何从synchronized了解Java对象头中的锁》中，咱们介绍并体验了JOL工具。尽管JOL很赞，但它的输入对咱们不是很敌对，如果不借助工具，咱们很难直观了解其中的含意。
上面这段代码是对JOL输入的翻译，倡议你珍藏。代码非我所写，文末曾经注明出处。
import org.openjdk.jol.info.ClassLayout;import java.nio.ByteOrder;import java.util.ArrayList;import java.util.Collections;import java.util.List;import java.util.regex.Matcher;import java.util.regex.Pattern;public class PrintObjectHeader { /** * Get binary data * * @param o * @return */ public static String getObjectHeader(Object o) { ByteOrder order = ByteOrder.nativeOrder();//Byte order String table = ClassLayout.parseInstance(o).toPrintable(); Pattern p = Pattern.compile("(0|1){8}"); Matcher matcher = p.matcher(table); List<String> header = new ArrayList<String>(); while (matcher.find()) { header.add(matcher.group()); } //Little-endian machines, need to traverse in reverse StringBuilder sb = new StringBuilder(); if (order.equals(ByteOrder.LITTLE_ENDIAN)) { Collections.reverse(header); } for (String s : header) { sb.append(s).append(" "); } return sb.toString().trim(); } /** * Parsing object header function for 64bit jvm * In 64bit jvm, the object header has two parts: Mark Word and Class Pointer, Mark Word takes 8 bytes, Class Pointer takes 4 bytes * * @param s Binary string of object header (each 8 bits, separated by a space) */ public static void parseObjectHeader(String s) { String[] tmp = s.split(" "); System.out.print("Class Pointer: "); for (int i = 0; i < 4; ++i) { System.out.print(tmp[i] + " "); } System.out.println("\nMark Word:"); if (tmp[11].charAt(5) == '0' && tmp[11].substring(6).equals("01")) {//0 01 lock-free state, regardless of GC mark //notice: Mark word structure without lock: unused(25bit) + hashcode(31bit) + unused(1bit) + age(4bit) + biased_lock_flag(1bit) + lock_type(2bit) // The reason why hashcode only needs 31bit is: hashcode can only be greater than or equal to 0, eliminating the negative range, so you can use 31bit to store System.out.print("\thashcode (31bit): "); System.out.print(tmp[7].substring(1) + " "); for (int i = 8; i < 11; ++i) System.out.print(tmp[i] + " "); System.out.println(); } else if (tmp[11].charAt(5) == '1' && tmp[11].substring(6).equals("01")) {//1 01, which is the case of biased lock //notice: The object is in a biased lock, its structure is: ThreadID(54bit) + epoch(2bit) + unused(1bit) + age(4bit) + biased_lock_flag(1bit) + lock_type(2bit) // ThreadID here is the thread ID holding the biased lock, epoch: a timestamp of the biased lock, used for optimization of the biased lock System.out.print("\tThreadID(54bit): "); for (int i = 4; i < 10; ++i) System.out.print(tmp[i] + " "); System.out.println(tmp[10].substring(0, 6)); System.out.println("\tepoch: " + tmp[10].substring(6)); } else {//In the case of lightweight locks or heavyweight locks, regardless of the GC mark //notice: JavaThread*(62bit,include zero padding) + lock_type(2bit) // At this point, JavaThread* points to the monitor of the lock record/heavyweight lock in the stack System.out.print("\tjavaThread*(62bit,include zero padding): "); for (int i = 4; i < 11; ++i) System.out.print(tmp[i] + " "); System.out.println(tmp[11].substring(0, 6)); System.out.println("\tLockFlag (2bit): " + tmp[11].substring(6)); System.out.println(); return; } System.out.println("\tage (4bit): " + tmp[11].substring(1, 5)); System.out.println("\tbiasedLockFlag (1bit): " + tmp[11].charAt(5)); System.out.println("\tLockFlag (2bit): " + tmp[11].substring(6)); System.out.println(); } public static void printObjectHeader(Object o) { if (o == null) { System.out.println("null object."); return; } parseObjectHeader(getObjectHeader(o)); }}参考资料
- https://www.programmersought....
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