关于kafka:一个诡异的Kafka消息OOM问题趁机扒一扒StringBuilder

1. 前言

  起源是某个客户线上环境的一次OOM，据说这种诡异的OOM曾经第二次了。首先形容一下OOM的相干信息：

1. 堆内存13G；
2. 应用G1垃圾收集器；
3. OOM后主动生成的dump文件7G；
4. 利用大抵性能是生产kafka数据，通过一些业务解决而后再次发回kafka【单条音讯据说32M左右】

客户的疑点：

1. 堆内存13G，dump文件7G，明明内存是够的，怎么就OOM了？

因为线上环境问题，不能随便复盘调试，只有一份内存Dump文件能够剖析。

2. 线上Dump文件剖析

2.1 问题点1 - kafka的RecordAccumulator占用大量内存

  此处无截图（遗记了），脑补。一度狐疑是kafka的本地缓存导致的OOM。个别状况RecordAccumulator和KafkaProducer是一一对应的【KafkaProducer每次初始化的时候在结构行数中创立RecordAccumulator实例】。然而客户说KafkaProducer是存在ThreadLocal中的，不会无限度创立，即RecordAccumulator也不会无限度创立。

  先保留此处问题可能性（而且RecordAccumulator透露的话也无法解释：明明内存是够的，怎么就OOM了？这个疑点）。

2.2 问题点1 - 线程栈里的OOM信息

  发现kafka producer的业务线程栈有OOM的内容：

2.3 问题点2 - 线程栈中有kafka音讯发送失败的信息

• kafka producer回调函数中呈现RecordTooLargeExeception报错

• kafka producer开始记录谬误日志

2.4 问题点3 - RecordTooLargeExeception

  剖析完一圈内存Dump，我给了本人一个大胆的猜测，会不会是理论的堆内存够的，而是触发了某个别的什么条件，导致显示的抛出了OutOfMemoryError的报错。

  带着这个纳闷，还是把注意力放在了线程栈上，再认真一看，忽然悟出了点什么。线程栈中是打印谬误日志的过程中抛出的OOM报错。而且是在构建日志音讯的过程中进行字符串拼接，进而进行StringBuilder扩容引起的数组拷贝，而后就是OOM了。至此，我把留神里转移到了StringBuilder上。

3. 扒一扒StringBuilder扩容

  带着下面所以的猜测，认真翻阅了一下StringBuilder中append()的源码以及扩容机制。

  在StringBuilder中，有一个字符数组叫value【申明在父类AbstractStringBuilder中】，这个才是用来寄存字符串内容的（在String类中同样有）。

StringBuilder.append()办法最终会调用父类的java.lang.AbstractStringBuilder#append(java.lang.String)办法，源码如下：

public AbstractStringBuilder append(String str) {        if (str == null)            return appendNull();        // 获取要追加的字符串长度        int len = str.length();        // 查看StringBuilder中字符数组的容量，以确保能够追加胜利        ensureCapacityInternal(count + len);        str.getChars(0, len, value, count);        count += len;        return this;    }

接下来看ensureCapacityInternal()办法：

能够看到，如果新字符串长度大于原来的字符串长度就会进行一次数组的拷贝。而且留神下面的正文：如果数值溢出，会抛出OutOfMemoryError。感觉我的猜测马上要被印证了，冲动ing。

认真看newCapacity()办法，

    /**     * The maximum size of array to allocate (unless necessary).     * Some VMs reserve some header words in an array.     * Attempts to allocate larger arrays may result in     * OutOfMemoryError: Requested array size exceeds VM limit     */    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;    /**     * Returns a capacity at least as large as the given minimum capacity.     * Returns the current capacity increased by the same amount + 2 if     * that suffices.     * Will not return a capacity greater than {@code MAX_ARRAY_SIZE}     * unless the given minimum capacity is greater than that.     *     * @param  minCapacity the desired minimum capacity     * @throws OutOfMemoryError if minCapacity is less than zero or     *         greater than Integer.MAX_VALUE     */    private int newCapacity(int minCapacity) {        // overflow-conscious code        // 新数组的长度为原来数组长度的2倍➕2        int newCapacity = (value.length << 1) + 2;        // 如果追加新字符所需起码的数组长度 大于 newCapacity，则将minCapacity的值赋给newCapacity        // 这种状况只呈现在追加的字符串长度是原字符串长度的2倍多的多        if (newCapacity - minCapacity < 0) {            newCapacity = minCapacity;        }        // 如果newCapacity大于Integer.MAX_VALUE - 8，则走hugeCapacity的逻辑        // 否则间接返回newCapacity        return (newCapacity <= 0 || MAX_ARRAY_SIZE - newCapacity < 0)            ? hugeCapacity(minCapacity)            : newCapacity;    }

MAX_ARRAY_SIZE的值是Integer.MAX_VALUE - 8。并且正文给了提醒：这是数组最大的长度了（除非必要），调配大数组可能导致OOM。

再看hugeCapacity()办法，

    private int hugeCapacity(int minCapacity) {        // 如果追加新字符所需起码的数组长度 大于Integer.MAX_VALUE，间接OOM        if (Integer.MAX_VALUE - minCapacity < 0) { // overflow            throw new OutOfMemoryError();        }        /**        * 如果追加新字符所需起码的数组长度 大于Integer.MAX_VALUE - 8，        * 则新数组长度是minCapacity，否则数组长度就是MAX_ARRAY_SIZE的值        * 在minCapacity大于Integer.MAX_VALUE一半的时候的最近一次扩容，        * 新数组长度会间接应用MAX_ARRAY_SIZE        **/        return (minCapacity > MAX_ARRAY_SIZE)            ? minCapacity : MAX_ARRAY_SIZE;    }

• 重点一句话总结下：字符串长度大于Integer.MAX_VALUE就会间接OOM。

那问题又来了：

1. 32M的音讯，再怎么扩容也不会超过Integer.MAX_VALUE的值吧；
2. 看上面的demo，这尼玛和Integer.MAX_VALUE差太多了吧；
   

莫慌，咱们持续（手摇轮椅上路）。

先说论断，这个其实和以后可用堆内存有关系。

4. 对象调配

• 先放一组数据：Integer.MAX_VALUE大小的字符数组大略须要占用4G堆内存（2147483646*2/1024/1024/1024）。

  先看下我本地做的两个试验，顺便温习一下GC log。

1. 启动参数（GC没配置，jdk8默认PS垃圾回收）：-Xmx8g -Xms8g -XX:+PrintGCDetails -XX:+PrintHeapAtGC -Xloggc:gc.log

✨GC日志如下：

Java HotSpot(TM) 64-Bit Server VM (25.281-b09) for bsd-amd64 JRE (1.8.0_281-b09), built on Dec  9 2020 12:44:49 by "java_re" with gcc 4.2.1 Compatible Apple LLVM 10.0.0 (clang-1000.11.45.5)Memory: 4k page, physical 33554432k(1613060k free)/proc/meminfo:CommandLine flags: -XX:InitialHeapSize=8589934592 -XX:MaxHeapSize=8589934592 -XX:+PrintGC -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintHeapAtGC -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelGC {Heap before GC invocations=1 (full 0): PSYoungGen      total 2446848K, used 1436457K [0x0000000715580000, 0x00000007c0000000, 0x00000007c0000000)  eden space 2097664K, 68% used [0x0000000715580000,0x000000076d04a5f8,0x0000000795600000)  from space 349184K, 0% used [0x00000007aab00000,0x00000007aab00000,0x00000007c0000000)  to   space 349184K, 0% used [0x0000000795600000,0x0000000795600000,0x00000007aab00000) ParOldGen       total 5592576K, used 3538944K [0x00000005c0000000, 0x0000000715580000, 0x0000000715580000)  object space 5592576K, 63% used [0x00000005c0000000,0x0000000698000020,0x0000000715580000) Metaspace       used 3918K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K7.302: [GC (Allocation Failure) [PSYoungGen: 1436457K->1444K(2446848K)] 4975401K->3540396K(8039424K), 0.0133225 secs] [Times: user=0.16 sys=0.00, real=0.01 secs] Heap after GC invocations=1 (full 0): PSYoungGen      total 2446848K, used 1444K [0x0000000715580000, 0x00000007c0000000, 0x00000007c0000000)  eden space 2097664K, 0% used [0x0000000715580000,0x0000000715580000,0x0000000795600000)  from space 349184K, 0% used [0x0000000795600000,0x00000007957691e0,0x00000007aab00000)  to   space 349184K, 0% used [0x00000007aab00000,0x00000007aab00000,0x00000007c0000000) ParOldGen       total 5592576K, used 3538952K [0x00000005c0000000, 0x0000000715580000, 0x0000000715580000)  object space 5592576K, 63% used [0x00000005c0000000,0x0000000698002020,0x0000000715580000) Metaspace       used 3918K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K}{Heap before GC invocations=2 (full 0): PSYoungGen      total 2446848K, used 1444K [0x0000000715580000, 0x00000007c0000000, 0x00000007c0000000)  eden space 2097664K, 0% used [0x0000000715580000,0x0000000715580000,0x0000000795600000)  from space 349184K, 0% used [0x0000000795600000,0x00000007957691e0,0x00000007aab00000)  to   space 349184K, 0% used [0x00000007aab00000,0x00000007aab00000,0x00000007c0000000) ParOldGen       total 5592576K, used 3538952K [0x00000005c0000000, 0x0000000715580000, 0x0000000715580000)  object space 5592576K, 63% used [0x00000005c0000000,0x0000000698002020,0x0000000715580000) Metaspace       used 3918K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K7.316: [GC (Allocation Failure) [PSYoungGen: 1444K->1491K(2446848K)] 3540396K->3540451K(8039424K), 0.0185590 secs] [Times: user=0.23 sys=0.00, real=0.02 secs] Heap after GC invocations=2 (full 0): PSYoungGen      total 2446848K, used 1491K [0x0000000715580000, 0x00000007c0000000, 0x00000007c0000000)  eden space 2097664K, 0% used [0x0000000715580000,0x0000000715580000,0x0000000795600000)  from space 349184K, 0% used [0x00000007aab00000,0x00000007aac74db8,0x00000007c0000000)  to   space 349184K, 0% used [0x0000000795600000,0x0000000795600000,0x00000007aab00000) ParOldGen       total 5592576K, used 3538960K [0x00000005c0000000, 0x0000000715580000, 0x0000000715580000)  object space 5592576K, 63% used [0x00000005c0000000,0x0000000698004020,0x0000000715580000) Metaspace       used 3918K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K}{Heap before GC invocations=3 (full 1): PSYoungGen      total 2446848K, used 1491K [0x0000000715580000, 0x00000007c0000000, 0x00000007c0000000)  eden space 2097664K, 0% used [0x0000000715580000,0x0000000715580000,0x0000000795600000)  from space 349184K, 0% used [0x00000007aab00000,0x00000007aac74db8,0x00000007c0000000)  to   space 349184K, 0% used [0x0000000795600000,0x0000000795600000,0x00000007aab00000) ParOldGen       total 5592576K, used 3538960K [0x00000005c0000000, 0x0000000715580000, 0x0000000715580000)  object space 5592576K, 63% used [0x00000005c0000000,0x0000000698004020,0x0000000715580000) Metaspace       used 3918K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K7.334: [Full GC (Allocation Failure) [PSYoungGen: 1491K->0K(2446848K)] [ParOldGen: 3538960K->2360557K(5592576K)] 3540451K->2360557K(8039424K), [Metaspace: 3918K->3918K(1056768K)], 0.2781291 secs] [Times: user=2.89 sys=0.16, real=0.28 secs] Heap after GC invocations=3 (full 1): PSYoungGen      total 2446848K, used 0K [0x0000000715580000, 0x00000007c0000000, 0x00000007c0000000)  eden space 2097664K, 0% used [0x0000000715580000,0x0000000715580000,0x0000000795600000)  from space 349184K, 0% used [0x00000007aab00000,0x00000007aab00000,0x00000007c0000000)  to   space 349184K, 0% used [0x0000000795600000,0x0000000795600000,0x00000007aab00000) ParOldGen       total 5592576K, used 2360557K [0x00000005c0000000, 0x0000000715580000, 0x0000000715580000)  object space 5592576K, 42% used [0x00000005c0000000,0x000000065013b760,0x0000000715580000) Metaspace       used 3918K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K}{Heap before GC invocations=4 (full 1): PSYoungGen      total 2446848K, used 0K [0x0000000715580000, 0x00000007c0000000, 0x00000007c0000000)  eden space 2097664K, 0% used [0x0000000715580000,0x0000000715580000,0x0000000795600000)  from space 349184K, 0% used [0x00000007aab00000,0x00000007aab00000,0x00000007c0000000)  to   space 349184K, 0% used [0x0000000795600000,0x0000000795600000,0x00000007aab00000) ParOldGen       total 5592576K, used 2360557K [0x00000005c0000000, 0x0000000715580000, 0x0000000715580000)  object space 5592576K, 42% used [0x00000005c0000000,0x000000065013b760,0x0000000715580000) Metaspace       used 3918K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K7.613: [GC (Allocation Failure) [PSYoungGen: 0K->0K(2446848K)] 2360557K->2360557K(8039424K), 0.0177683 secs] [Times: user=0.22 sys=0.00, real=0.01 secs] Heap after GC invocations=4 (full 1): PSYoungGen      total 2446848K, used 0K [0x0000000715580000, 0x00000007c0000000, 0x00000007c0000000)  eden space 2097664K, 0% used [0x0000000715580000,0x0000000715580000,0x0000000795600000)  from space 349184K, 0% used [0x0000000795600000,0x0000000795600000,0x00000007aab00000)  to   space 349184K, 0% used [0x00000007aab00000,0x00000007aab00000,0x00000007c0000000) ParOldGen       total 5592576K, used 2360557K [0x00000005c0000000, 0x0000000715580000, 0x0000000715580000)  object space 5592576K, 42% used [0x00000005c0000000,0x000000065013b760,0x0000000715580000) Metaspace       used 3918K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K}{Heap before GC invocations=5 (full 2): PSYoungGen      total 2446848K, used 0K [0x0000000715580000, 0x00000007c0000000, 0x00000007c0000000)  eden space 2097664K, 0% used [0x0000000715580000,0x0000000715580000,0x0000000795600000)  from space 349184K, 0% used [0x0000000795600000,0x0000000795600000,0x00000007aab00000)  to   space 349184K, 0% used [0x00000007aab00000,0x00000007aab00000,0x00000007c0000000) ParOldGen       total 5592576K, used 2360557K [0x00000005c0000000, 0x0000000715580000, 0x0000000715580000)  object space 5592576K, 42% used [0x00000005c0000000,0x000000065013b760,0x0000000715580000) Metaspace       used 3918K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K7.630: [Full GC (Allocation Failure) [PSYoungGen: 0K->0K(2446848K)] [ParOldGen: 2360557K->2360504K(5592576K)] 2360557K->2360504K(8039424K), [Metaspace: 3918K->3918K(1056768K)], 0.0095262 secs] [Times: user=0.06 sys=0.00, real=0.01 secs] Heap after GC invocations=5 (full 2): PSYoungGen      total 2446848K, used 0K [0x0000000715580000, 0x00000007c0000000, 0x00000007c0000000)  eden space 2097664K, 0% used [0x0000000715580000,0x0000000715580000,0x0000000795600000)  from space 349184K, 0% used [0x0000000795600000,0x0000000795600000,0x00000007aab00000)  to   space 349184K, 0% used [0x00000007aab00000,0x00000007aab00000,0x00000007c0000000) ParOldGen       total 5592576K, used 2360504K [0x00000005c0000000, 0x0000000715580000, 0x0000000715580000)  object space 5592576K, 42% used [0x00000005c0000000,0x000000065012e1d8,0x0000000715580000) Metaspace       used 3918K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K}Heap PSYoungGen      total 2446848K, used 52441K [0x0000000715580000, 0x00000007c0000000, 0x00000007c0000000)  eden space 2097664K, 2% used [0x0000000715580000,0x00000007188b67e8,0x0000000795600000)  from space 349184K, 0% used [0x0000000795600000,0x0000000795600000,0x00000007aab00000)  to   space 349184K, 0% used [0x00000007aab00000,0x00000007aab00000,0x00000007c0000000) ParOldGen       total 5592576K, used 2360504K [0x00000005c0000000, 0x0000000715580000, 0x0000000715580000)  object space 5592576K, 42% used [0x00000005c0000000,0x000000065012e1d8,0x0000000715580000) Metaspace       used 3926K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 430K, capacity 460K, committed 512K, reserved 1048576K

  能够看到最初通过Full GC后，字符数组被挪到了老年代，此时老年代占用2360504K，正好是长度为1207959551的字符数组的大小。此时如果要再进行扩容，新数组长度就是Integer.MAX_VALUE - 8，也就是须要在堆内存再调配将近4G的数组，然而不论是年老代还是老年代，都容不下这4G大小，因而也就OOM了。

2. 启动参数（GC没配置，jdk8默认PS垃圾回收）：-Xmx16g -Xms16g -XX:+PrintGCDetails -XX:+PrintHeapAtGC -Xloggc:gc.log
   【eden：5588992K ParOldGen：11185152K】

✨GC日志如下：

Java HotSpot(TM) 64-Bit Server VM (25.281-b09) for bsd-amd64 JRE (1.8.0_281-b09), built on Dec  9 2020 12:44:49 by "java_re" with gcc 4.2.1 Compatible Apple LLVM 10.0.0 (clang-1000.11.45.5)Memory: 4k page, physical 33554432k(942232k free)/proc/meminfo:CommandLine flags: -XX:InitialHeapSize=17179869184 -XX:MaxHeapSize=17179869184 -XX:+PrintGC -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintHeapAtGC -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelGC {Heap before GC invocations=1 (full 0): PSYoungGen      total 4893184K, used 2788885K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 4194304K, 66% used [0x000000066ab00000,0x0000000714e85418,0x000000076ab00000)  from space 698880K, 0% used [0x0000000795580000,0x0000000795580000,0x00000007c0000000)  to   space 698880K, 0% used [0x000000076ab00000,0x000000076ab00000,0x0000000795580000) ParOldGen       total 11185152K, used 10747904K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 96% used [0x00000003c0000000,0x0000000650000010,0x000000066ab00000) Metaspace       used 3914K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K18.887: [GC (Allocation Failure) [PSYoungGen: 2788885K->1488K(4893184K)] 13536789K->10749400K(16078336K), 0.0556800 secs] [Times: user=0.39 sys=0.21, real=0.05 secs] Heap after GC invocations=1 (full 0): PSYoungGen      total 4893184K, used 1488K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 4194304K, 0% used [0x000000066ab00000,0x000000066ab00000,0x000000076ab00000)  from space 698880K, 0% used [0x000000076ab00000,0x000000076ac74010,0x0000000795580000)  to   space 698880K, 0% used [0x0000000795580000,0x0000000795580000,0x00000007c0000000) ParOldGen       total 11185152K, used 10747912K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 96% used [0x00000003c0000000,0x0000000650002010,0x000000066ab00000) Metaspace       used 3914K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K}{Heap before GC invocations=2 (full 1): PSYoungGen      total 4893184K, used 1488K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 4194304K, 0% used [0x000000066ab00000,0x000000066ab00000,0x000000076ab00000)  from space 698880K, 0% used [0x000000076ab00000,0x000000076ac74010,0x0000000795580000)  to   space 698880K, 0% used [0x0000000795580000,0x0000000795580000,0x00000007c0000000) ParOldGen       total 11185152K, used 10747912K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 96% used [0x00000003c0000000,0x0000000650002010,0x000000066ab00000) Metaspace       used 3914K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K18.943: [Full GC (Ergonomics) [PSYoungGen: 1488K->0K(4893184K)] [ParOldGen: 10747912K->4195565K(11185152K)] 10749400K->4195565K(16078336K), [Metaspace: 3914K->3914K(1056768K)], 0.9751963 secs] [Times: user=2.33 sys=4.18, real=0.98 secs] Heap after GC invocations=2 (full 1): PSYoungGen      total 4893184K, used 0K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 4194304K, 0% used [0x000000066ab00000,0x000000066ab00000,0x000000076ab00000)  from space 698880K, 0% used [0x000000076ab00000,0x000000076ab00000,0x0000000795580000)  to   space 698880K, 0% used [0x0000000795580000,0x0000000795580000,0x00000007c0000000) ParOldGen       total 11185152K, used 4195565K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 37% used [0x00000003c0000000,0x00000004c013b720,0x000000066ab00000) Metaspace       used 3914K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K}{Heap before GC invocations=3 (full 1): PSYoungGen      total 4893184K, used 0K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 4194304K, 0% used [0x000000066ab00000,0x000000066ab00000,0x000000076ab00000)  from space 698880K, 0% used [0x000000076ab00000,0x000000076ab00000,0x0000000795580000)  to   space 698880K, 0% used [0x0000000795580000,0x0000000795580000,0x00000007c0000000) ParOldGen       total 11185152K, used 8389869K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 75% used [0x00000003c0000000,0x00000005c013b728,0x000000066ab00000) Metaspace       used 3916K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K20.945: [GC (Allocation Failure) [PSYoungGen: 0K->0K(4893184K)] 8389869K->8389869K(16078336K), 0.0296937 secs] [Times: user=0.38 sys=0.00, real=0.03 secs] Heap after GC invocations=3 (full 1): PSYoungGen      total 4893184K, used 0K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 4194304K, 0% used [0x000000066ab00000,0x000000066ab00000,0x000000076ab00000)  from space 698880K, 0% used [0x0000000795580000,0x0000000795580000,0x00000007c0000000)  to   space 698880K, 0% used [0x000000076ab00000,0x000000076ab00000,0x0000000795580000) ParOldGen       total 11185152K, used 8389869K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 75% used [0x00000003c0000000,0x00000005c013b728,0x000000066ab00000) Metaspace       used 3916K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K}{Heap before GC invocations=4 (full 1): PSYoungGen      total 4893184K, used 0K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 4194304K, 0% used [0x000000066ab00000,0x000000066ab00000,0x000000076ab00000)  from space 698880K, 0% used [0x0000000795580000,0x0000000795580000,0x00000007c0000000)  to   space 698880K, 0% used [0x000000076ab00000,0x000000076ab00000,0x0000000795580000) ParOldGen       total 11185152K, used 8389869K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 75% used [0x00000003c0000000,0x00000005c013b728,0x000000066ab00000) Metaspace       used 3916K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K20.975: [GC (Allocation Failure) [PSYoungGen: 0K->0K(4893184K)] 8389869K->8389869K(16078336K), 0.0293211 secs] [Times: user=0.37 sys=0.00, real=0.04 secs] Heap after GC invocations=4 (full 1): PSYoungGen      total 4893184K, used 0K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 4194304K, 0% used [0x000000066ab00000,0x000000066ab00000,0x000000076ab00000)  from space 698880K, 0% used [0x000000076ab00000,0x000000076ab00000,0x0000000795580000)  to   space 698880K, 0% used [0x0000000795580000,0x0000000795580000,0x00000007c0000000) ParOldGen       total 11185152K, used 8389869K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 75% used [0x00000003c0000000,0x00000005c013b728,0x000000066ab00000) Metaspace       used 3916K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K}{Heap before GC invocations=5 (full 2): PSYoungGen      total 4893184K, used 0K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 4194304K, 0% used [0x000000066ab00000,0x000000066ab00000,0x000000076ab00000)  from space 698880K, 0% used [0x000000076ab00000,0x000000076ab00000,0x0000000795580000)  to   space 698880K, 0% used [0x0000000795580000,0x0000000795580000,0x00000007c0000000) ParOldGen       total 11185152K, used 8389869K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 75% used [0x00000003c0000000,0x00000005c013b728,0x000000066ab00000) Metaspace       used 3916K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K21.019: [Full GC (Allocation Failure) [PSYoungGen: 0K->0K(4893184K)] [ParOldGen: 8389869K->4195332K(11185152K)] 8389869K->4195332K(16078336K), [Metaspace: 3916K->3916K(1056768K)], 0.4611617 secs] [Times: user=5.40 sys=0.04, real=0.46 secs] Heap after GC invocations=5 (full 2): PSYoungGen      total 4893184K, used 0K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 4194304K, 0% used [0x000000066ab00000,0x000000066ab00000,0x000000076ab00000)  from space 698880K, 0% used [0x000000076ab00000,0x000000076ab00000,0x0000000795580000)  to   space 698880K, 0% used [0x0000000795580000,0x0000000795580000,0x00000007c0000000) ParOldGen       total 11185152K, used 4195332K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 37% used [0x00000003c0000000,0x00000004c0101090,0x000000066ab00000) Metaspace       used 3916K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K}{Heap before GC invocations=6 (full 2): PSYoungGen      total 4893184K, used 83886K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 4194304K, 2% used [0x000000066ab00000,0x000000066fceb868,0x000000076ab00000)  from space 698880K, 0% used [0x000000076ab00000,0x000000076ab00000,0x0000000795580000)  to   space 698880K, 0% used [0x0000000795580000,0x0000000795580000,0x00000007c0000000) ParOldGen       total 11185152K, used 8389636K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 75% used [0x00000003c0000000,0x00000005c0101098,0x000000066ab00000) Metaspace       used 3916K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K21.963: [GC (Allocation Failure) [PSYoungGen: 83886K->0K(4893184K)] 8473522K->8389636K(16078336K), 0.0356100 secs] [Times: user=0.46 sys=0.00, real=0.04 secs] Heap after GC invocations=6 (full 2): PSYoungGen      total 4893184K, used 0K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 4194304K, 0% used [0x000000066ab00000,0x000000066ab00000,0x000000076ab00000)  from space 698880K, 0% used [0x0000000795580000,0x0000000795580000,0x00000007c0000000)  to   space 698880K, 0% used [0x000000076ab00000,0x000000076ab00000,0x0000000795580000) ParOldGen       total 11185152K, used 8389636K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 75% used [0x00000003c0000000,0x00000005c0101098,0x000000066ab00000) Metaspace       used 3916K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K}{Heap before GC invocations=7 (full 2): PSYoungGen      total 4893184K, used 0K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 4194304K, 0% used [0x000000066ab00000,0x000000066ab00000,0x000000076ab00000)  from space 698880K, 0% used [0x0000000795580000,0x0000000795580000,0x00000007c0000000)  to   space 698880K, 0% used [0x000000076ab00000,0x000000076ab00000,0x0000000795580000) ParOldGen       total 11185152K, used 8389636K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 75% used [0x00000003c0000000,0x00000005c0101098,0x000000066ab00000) Metaspace       used 3916K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K21.999: [GC (Allocation Failure) [PSYoungGen: 0K->0K(4893184K)] 8389636K->8389636K(16078336K), 0.0234444 secs] [Times: user=0.30 sys=0.00, real=0.02 secs] Heap after GC invocations=7 (full 2): PSYoungGen      total 4893184K, used 0K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 4194304K, 0% used [0x000000066ab00000,0x000000066ab00000,0x000000076ab00000)  from space 698880K, 0% used [0x000000076ab00000,0x000000076ab00000,0x0000000795580000)  to   space 1536K, 0% used [0x00000007bfe80000,0x00000007bfe80000,0x00000007c0000000) ParOldGen       total 11185152K, used 8389636K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 75% used [0x00000003c0000000,0x00000005c0101098,0x000000066ab00000) Metaspace       used 3916K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K}{Heap before GC invocations=8 (full 3): PSYoungGen      total 4893184K, used 0K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 4194304K, 0% used [0x000000066ab00000,0x000000066ab00000,0x000000076ab00000)  from space 698880K, 0% used [0x000000076ab00000,0x000000076ab00000,0x0000000795580000)  to   space 1536K, 0% used [0x00000007bfe80000,0x00000007bfe80000,0x00000007c0000000) ParOldGen       total 11185152K, used 8389636K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 75% used [0x00000003c0000000,0x00000005c0101098,0x000000066ab00000) Metaspace       used 3916K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K22.023: [Full GC (Allocation Failure) [PSYoungGen: 0K->0K(4893184K)] [ParOldGen: 8389636K->4195323K(11185152K)] 8389636K->4195323K(16078336K), [Metaspace: 3916K->3916K(1056768K)], 0.4557171 secs] [Times: user=5.56 sys=0.04, real=0.46 secs] Heap after GC invocations=8 (full 3): PSYoungGen      total 4893184K, used 0K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 4194304K, 0% used [0x000000066ab00000,0x000000066ab00000,0x000000076ab00000)  from space 698880K, 0% used [0x000000076ab00000,0x000000076ab00000,0x0000000795580000)  to   space 1536K, 0% used [0x00000007bfe80000,0x00000007bfe80000,0x00000007c0000000) ParOldGen       total 11185152K, used 4195323K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 37% used [0x00000003c0000000,0x00000004c00fedf8,0x000000066ab00000) Metaspace       used 3916K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K}{Heap before GC invocations=9 (full 3): PSYoungGen      total 4893184K, used 0K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 4194304K, 0% used [0x000000066ab00000,0x000000066ab00000,0x000000076ab00000)  from space 698880K, 0% used [0x000000076ab00000,0x000000076ab00000,0x0000000795580000)  to   space 1536K, 0% used [0x00000007bfe80000,0x00000007bfe80000,0x00000007c0000000) ParOldGen       total 11185152K, used 8389627K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 75% used [0x00000003c0000000,0x00000005c00fee00,0x000000066ab00000) Metaspace       used 3916K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K22.976: [GC (Allocation Failure) [PSYoungGen: 0K->0K(5590528K)] 8389627K->8389627K(16775680K), 0.0401370 secs] [Times: user=0.52 sys=0.00, real=0.04 secs] Heap after GC invocations=9 (full 3): PSYoungGen      total 5590528K, used 0K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 5588992K, 0% used [0x000000066ab00000,0x000000066ab00000,0x00000007bfd00000)  from space 1536K, 0% used [0x00000007bfe80000,0x00000007bfe80000,0x00000007c0000000)  to   space 1536K, 0% used [0x00000007bfd00000,0x00000007bfd00000,0x00000007bfe80000) ParOldGen       total 11185152K, used 8389627K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 75% used [0x00000003c0000000,0x00000005c00fee00,0x000000066ab00000) Metaspace       used 3916K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K}{Heap before GC invocations=10 (full 3): PSYoungGen      total 5590528K, used 4194304K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 5588992K, 75% used [0x000000066ab00000,0x000000076ab00008,0x00000007bfd00000)  from space 1536K, 0% used [0x00000007bfe80000,0x00000007bfe80000,0x00000007c0000000)  to   space 1536K, 0% used [0x00000007bfd00000,0x00000007bfd00000,0x00000007bfe80000) ParOldGen       total 11185152K, used 8389627K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 75% used [0x00000003c0000000,0x00000005c00fee00,0x000000066ab00000) Metaspace       used 3916K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K25.776: [GC (Allocation Failure) --[PSYoungGen: 4194304K->4194304K(5590528K)] 12583931K->12583931K(16775680K), 0.0263122 secs] [Times: user=0.32 sys=0.00, real=0.02 secs] Heap after GC invocations=10 (full 3): PSYoungGen      total 5590528K, used 4194304K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 5588992K, 75% used [0x000000066ab00000,0x000000076ab00008,0x00000007bfd00000)  from space 1536K, 0% used [0x00000007bfe80000,0x00000007bfe80000,0x00000007c0000000)  to   space 1536K, 0% used [0x00000007bfd00000,0x00000007bfd00000,0x00000007bfe80000) ParOldGen       total 11185152K, used 8389627K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 75% used [0x00000003c0000000,0x00000005c00fee00,0x000000066ab00000) Metaspace       used 3916K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K}{Heap before GC invocations=11 (full 4): PSYoungGen      total 5590528K, used 4194304K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 5588992K, 75% used [0x000000066ab00000,0x000000076ab00008,0x00000007bfd00000)  from space 1536K, 0% used [0x00000007bfe80000,0x00000007bfe80000,0x00000007c0000000)  to   space 1536K, 0% used [0x00000007bfd00000,0x00000007bfd00000,0x00000007bfe80000) ParOldGen       total 11185152K, used 8389627K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 75% used [0x00000003c0000000,0x00000005c00fee00,0x000000066ab00000) Metaspace       used 3916K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K25.802: [Full GC (Ergonomics) [PSYoungGen: 4194304K->0K(5590528K)] [ParOldGen: 8389627K->4195328K(11185152K)] 12583931K->4195328K(16775680K), [Metaspace: 3916K->3916K(1056768K)], 0.8243356 secs] [Times: user=1.74 sys=1.38, real=0.83 secs] Heap after GC invocations=11 (full 4): PSYoungGen      total 5590528K, used 0K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 5588992K, 0% used [0x000000066ab00000,0x000000066ab00000,0x00000007bfd00000)  from space 1536K, 0% used [0x00000007bfe80000,0x00000007bfe80000,0x00000007c0000000)  to   space 1536K, 0% used [0x00000007bfd00000,0x00000007bfd00000,0x00000007bfe80000) ParOldGen       total 11185152K, used 4195328K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 37% used [0x00000003c0000000,0x00000004c0100008,0x000000066ab00000) Metaspace       used 3916K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 429K, capacity 460K, committed 512K, reserved 1048576K}Heap PSYoungGen      total 5590528K, used 4343344K [0x000000066ab00000, 0x00000007c0000000, 0x00000007c0000000)  eden space 5588992K, 77% used [0x000000066ab00000,0x0000000773c8c0c0,0x00000007bfd00000)  from space 1536K, 0% used [0x00000007bfe80000,0x00000007bfe80000,0x00000007c0000000)  to   space 1536K, 0% used [0x00000007bfd00000,0x00000007bfd00000,0x00000007bfe80000) ParOldGen       total 11185152K, used 4195328K [0x00000003c0000000, 0x000000066ab00000, 0x000000066ab00000)  object space 11185152K, 37% used [0x00000003c0000000,0x00000004c0100008,0x000000066ab00000) Metaspace       used 3924K, capacity 4568K, committed 4864K, reserved 1056768K  class space    used 430K, capacity 460K, committed 512K, reserved 1048576K

  能够看到老年代占用为4195328K，这个是扩容前字符数组所占大小，年老代eden区4194304K，是扩容后长度2147483646的新数组的大小。因而在16G的堆上通过几次Full GC的整顿，最终可能实现StringBuilder的顶格扩容。

须要留神的是⚠️：demo中只有一个StringBuilder，在理论环境中势必存在很多其余的对象，因而根本不存在
StringBuilder顶格扩容的状况。更多的是在StringBuilder扩容时，可能随同着其余对象的调配，而导致
Arrays#copyOf办法抛出OOM。

• 论断：

  1. StringBuilder在扩容的时候，会进行字符数组拷贝，在数组长度小于Integer.MAX_VALUE一半的状况下，会创立一个新数组用于拷贝，新数组长度是原数组的2倍➕2【此时内存占用：原数组大小➕2*原数组大小】；
  2. 如果数组超过Integer.MAX_VALUE的一半的时候的最近一次扩容，新数组长度间接应用MAX_ARRAY_SIZE（Integer.MAX_VALUE - 8）；
  3. 在16G堆内存默认比例划分的状况下，字符数组能力胜利扩容至Integer.MAX_VALUE长度（⚠️留神，这里的demo仅仅只有StringBuilder对象，理论状况必定还有很多其余对象，因而理论环境很少有StringBuilder能够实现顶格扩容）；
  4. 在扩容的过程中，新数组的大小如果eden区调配不下，则进行一次Young GC，如果还是无奈调配，则间接在老年代调配；如果老年待也调配不下，则进行Full GC【可能进行几轮Full GC】，之后还是调配不下则OOM（抛出的中央是在java.util.Arrays#copyOf(char[], int)的new char[newLength]创立新数组的时候）；