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How to format Java memory tool JOL output

Posted on Dec. 2, 2022, 6:50 p.m. by Amber Mccormick
Category: The back-end Tag: The back-end java

Welcome to The sixth article in a series on Concurrency.

In the previous article, "Inside Out: How to Understand Locks in Java Object Headers from Synchronized," we introduced and experimented with the JOL tool. While JOL is great, its output isn't very friendly to us, and it's hard to intuitively understand what it means without tools.

The following code is a translation of JOL output. I recommend you bookmark it. I did not write the code, the source is given at the end of the article.


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);
        ListString header = new ArrayListString();
        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)); }}Copy the code

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