摘要：一個(gè)對(duì)象內(nèi)存有多大為什么想知道這個(gè)，自以為很重要，其實(shí)。參考鏈接對(duì)象解析不得不了解的對(duì)象頭從開始在位系統(tǒng)上會(huì)默認(rèn)開啟壓縮指針研究位引用指針壓縮技術(shù)

為什么想知道這個(gè)，自以為很重要，其實(shí) just for fun =。=

jdk: java 官網(wǎng)上下載的，HotSpot 虛擬機(jī)

java version "1.8.0_121"
Java(TM) SE Runtime Environment (build 1.8.0_121-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.121-b13, mixed mode)

使用的方法： Instrumentation 參考的是 一個(gè)對(duì)象占用多少字節(jié)？

確認(rèn)編譯時(shí)是 32 位還是 64 位，對(duì)測(cè)試結(jié)果有影響。x86 為 32 位， amd64 為 64 位，其余我不知道

System.out.println(System.getProperty("os.arch"));

SizeOfObject.java 直接從上面鏈接上拷貝

編譯這個(gè)類，得到 .class 文件

在包名路徑下執(zhí)行一下命令進(jìn)行打包（注意修改相應(yīng)的包名）：

jar cvfm SizeOfObject.jar manifest.mf org/seal_de/SizeOfObject.class

其中 manifest.mf 清單信息為： PreMain-Class: org.seal_de.SizeOfObject

PS: 如果在打包好的 jar 包中，META-INF/MANIFEST.MF 沒有 PreMain-Class 屬性，添加上去即可

在運(yùn)行測(cè)試程序的時(shí)候，添加 vm 參數(shù)

-javaagent:{jar包路徑}SizeOfObject.jar

測(cè)試 int, Object, 引用的大小。其余類型測(cè)試都類似

public class MemoryTest {
    /**
     * -javaagent:{jar包路徑}SizeOfObject.jar -XX:+UseCompressedOops
     * 使用指針壓縮，在一定情況下64位HotSpot jvm默認(rèn)指針壓縮
     *
     *Output:
     *amd64
     *Object:                              16
     *    
     *include one int:                     16
     *include two int:                     24
     *include three int:                   24
     *
     *include one object:                  16
     *include one object:                  24
     */
    static void test1() {
        System.out.println(System.getProperty("os.arch"));
        System.out.printf("%-30s%9d
", "Object:", SizeOfObject.sizeOf(new Object()));
        System.out.println();
        System.out.printf("%-30s%9d
", "include one int:", SizeOfObject.sizeOf(new IntegerTestOne()));
        System.out.printf("%-30s%9d
", "include two int:", SizeOfObject.sizeOf(new IntegerTestTwo()));
        System.out.printf("%-30s%9d
", "include three int:", SizeOfObject.sizeOf(new IntegerTestThree()));
        System.out.println();
        System.out.printf("%-30s%9d
", "include one object:", SizeOfObject.sizeOf(new ReferenceTestOne()));
        System.out.printf("%-30s%9d
", "include one object:", SizeOfObject.sizeOf(new ReferenceTestTwo()));
    }

    /**
     * -javaagent:{jar包路徑}SizeOfObject.jar -XX:-UseCompressedOops
     * 不使用指針壓縮
     *
     *Output:
     *amd64
     *Object:                              16
     *
     *include one int:                     24
     *include two int:                     24
     *include three int:                   32
     *
     *include one object:                  24
     *include one object:                  32
     */
    static void test2() {
        System.out.println(System.getProperty("os.arch"));
        System.out.printf("%-30s%9d
", "Object:", SizeOfObject.sizeOf(new Object()));
        System.out.println();
        System.out.printf("%-30s%9d
", "include one int:", SizeOfObject.sizeOf(new IntegerTestOne()));
        System.out.printf("%-30s%9d
", "include two int:", SizeOfObject.sizeOf(new IntegerTestTwo()));
        System.out.printf("%-30s%9d
", "include three int:", SizeOfObject.sizeOf(new IntegerTestThree()));
        System.out.println();
        System.out.printf("%-30s%9d
", "include one object:", SizeOfObject.sizeOf(new ReferenceTestOne()));
        System.out.printf("%-30s%9d
", "include one object:", SizeOfObject.sizeOf(new ReferenceTestTwo()));
    }

    public static void main(String[] args) {
        test2();
    }

    static class IntegerTestOne {
        private int i1 = 1;
    }

    static class IntegerTestTwo {
        private int i1 = 1;
        private int i2 = 1;
    }

    static class IntegerTestThree {
        private int i1 = 1;
        private int i2 = 1;
        private int i3 = 1;
    }

    static class ReferenceTestOne {
        private Object o1 = new Object();
    }

    static class ReferenceTestTwo {
        private Object o1 = new Object();
        private Object o2 = new Object();
    }
}

對(duì)象內(nèi)存 = 對(duì)象頭 + 類型指針 + 對(duì)齊填充

對(duì)象頭不參與指針壓縮，并且 32 位時(shí)為 4 個(gè)字節(jié)，64 位時(shí)為 8 個(gè)字節(jié)

類型指針參與指針壓縮，并且 32 位時(shí)為 4 個(gè)字節(jié)，64 位時(shí)為 8 個(gè)字節(jié)；指針壓縮時(shí) 64 位為 4 個(gè)字節(jié)

對(duì)齊填充，由于 jvm 設(shè)計(jì)內(nèi)存要為 8 字節(jié)的整數(shù)倍，所以不足的需要填充。如 對(duì)象頭和類型指針一共 12 字節(jié)，填充后為 16 字節(jié)，填充了 4 個(gè)字節(jié)

其中 (8 + 8) 為對(duì)象頭和類型指針的字節(jié)數(shù)

public class MemoryStaticTest {
    /**
     * -javaagent:{jar包路徑}SizeOfObject.jar
     * 使用指針壓縮，在一定情況下64位HotSpot jvm默認(rèn)指針壓縮
     *
     * Output:
     * amd64    +UseCompressedOops
     * StaticTest:                          16
     * Integer:                             16
     * StaticReferenceTest:                 16
     *
     * @param args
     */
    public static void main(String[] args) {
        System.out.println(System.getProperty("os.arch") + "    +UseCompressedOops");
        System.out.printf("%-30s%9d
", "StaticTest:", SizeOfObject.sizeOf(new StaticTest()));
        System.out.printf("%-30s%9d
", "Integer:", SizeOfObject.sizeOf(new Integer(1)));
        System.out.printf("%-30s%9d
", "StaticReferenceTest:", SizeOfObject.sizeOf(new StaticReferenceTest()));
    }

    static class StaticTest {
        private static int i1 = 1;
        private static int i2 = 1;
        private static int i3 = 1;
        private static int i4 = 1;
        private static int i5 = 1;
        private static int i6 = 1;
    }

    static class StaticReferenceTest {
        private static Object o1 = new Object();
        private static Object o2 = new Object();
        private static Object o3 = new Object();
        private static Object o4 = new Object();
        private static Object o5 = new Object();

    }
}

代碼中還有一點(diǎn)其他測(cè)試： https://github.com/Deeeeeeeee/jcoding.git

對(duì)象頭在 64 位的時(shí)候，為什么是 8 個(gè)字節(jié)；32 位的時(shí)候，為什么是 4 個(gè)字節(jié)

一個(gè)引用在 64 位的時(shí)候，為什么是 8 個(gè)字節(jié)；32 位的時(shí)候，為什么是 4 個(gè)字節(jié)

這個(gè)跟 C++ 指針有關(guān)了，跟寄存器有關(guān)了

指針壓縮又是怎么個(gè)回事

這一切都可以在 jvm 的實(shí)現(xiàn)中找到答案，也就是看 C++ 代碼。只是我還沒到這個(gè)階段=。=

【Java對(duì)象解析】不得不了解的對(duì)象頭

JDK從6 update 23開始在64位系統(tǒng)上會(huì)默認(rèn)開啟壓縮指針

【Java JVM】 Hotspot GC研究- 64位引用指針壓縮技術(shù)