每日一面 - java中，描述一下什么情况下，对象会从年轻代进入老年代？

灰太狼 2023-01-01 13:54 110阅读 0赞

> 本问题参考自： https://www.zhihu.com/question/437632685， 解答为**个人原创**

# Key Takeaway #

*  Java **默认启用了分代 GC**
 *  启用分代 GC 的，在发生 Young GC，更准确地说是在 Survivor 区复制的时候，**存活的对象的分代年龄会加1**。
 *  当**分代年龄 = -XX:MaxTenuringThreshold** 指定的大小时，对象进入老年代
 *  还有动态晋升到老年代的机制，首先根据 **-XX:TargetSurvivorRatio** （默认 50，也就是 50%） 指定的比例，乘以 survivor 一个区的大小，得出**目标晋升空间大小**。然后将分代对象大小，**按照分代年龄从小到大相加，直到大于目标晋升空间大小**。之后，将**得出的这个分代年龄以上的对象全部晋升**。
 *  对于一些的 GC 算法，还可能直接在老年代上面分配，例如 G1 GC 中的 **humongous allocations（大对象分配）**，就是对象在超过 Region 一半大小的时候，直接在老年代的连续空间分配。

# 对象分配 #

我们一般认为 Java 中 new 的对象都是在堆上分配，这个说法不够准确，应该是大部分对象在堆上的 **TLAB分配**，还有一部分在 **栈上分配** 或者是 **堆上直接分配，可能 Eden 区也可能年老代**。同时，对于一些的 GC 算法，还可能直接在老年代上面分配，例如 G1 GC 中的 **humongous allocations（大对象分配）**，就是对象在超过 Region 一半大小的时候，直接在老年代的连续空间分配。  
这里，我们先只关心 TLAB 分配。  
对于单线程应用，每次分配内存，会记录上次分配对象内存地址末尾的指针，之后分配对象会从这个指针开始检索分配。这个机制叫做 **bump-the-pointer** （撞针）。  
对于多线程应用来说，内存分配需要考虑线程安全。最直接的想法就是通过全局锁，但是这个性能会很差。为了优化这个性能，我们考虑可以每个线程分配一个线程本地私有的内存池，然后采用 **bump-the-pointer** 机制进行内存分配。这个线程本地私有的内存池，就是 TLAB。只有 TLAB 满了，再去申请内存的时候，需要扩充 TLAB 或者使用新的 TLAB，这时候才需要锁。这样大大减少了锁使用。

![image][]

更详细的 TLAB 理解，请参考： [通过 JFR 与日志深入探索 JVM - TLAB 原理详解][JFR _ JVM - TLAB]

# 分代年龄 #

分代年龄位于对象头中，用于分代 GC.记录分代年龄一共 4 bit，所以最大为 2^4 - 1 = 15。所以配置最大分代年龄`-XX:MaxTenuringThreshold=n`这个n不能大于16，当然也不能小于 0.等于 0 的话，就直接入老年代。等于 16 的话（但是不能设置为 16 哟），就是从不进入老年代。默认是 15。

在发生 Young GC，更准确地说是在 Survivor 区复制的时候，存活的对象的分代年龄会加1。我们编写程序测试下，由于 编译器会优化代码，同时调用`System.gc()`并不是立刻触发 GC，并且是 Full GC，可能会使对象直接进入老年代，分代年龄不再增长，所以我们可以使用 volatile 属性辅助我们真正创建对象，避免编译器优化：

static volatile Object consumer;
    public static void main(String[] args) throws Exception {
        //这是我们要观察的对象
        Object instance = new Object();
        long lastAddr = VM.current().addressOf(instance);
        for (int i = 0; i < 10000; i++) {
            //查看地址是否发生了变化，代表是否发生了 Survivor 复制，或者是移动到老年代
            long currentAddr = VM.current().addressOf(instance);
            if (currentAddr != lastAddr) {
                //地址发生变化的时候，打印对象结构
                ClassLayout layout = ClassLayout.parseInstance(instance);
                System.out.println(layout.toPrintable());
                lastAddr = currentAddr;
            }
            for (int j = 0; j < 10000; j++) {
                //一直创建新对象
                //因为是volatile的属性更新，不会被编译器优化
                consumer = new Object();
            }
        }
    }

可以配合 GC 日志一起观察，关于 JVM 日志配置可以参考这篇文章：[OpenJDK 11 JVM日志相关参数解析与使用][OpenJDK 11 JVM]

首先我们用这个参数运行程序`-Xmx128m -Xlog:gc=info`，输出：

[0.016s][info][gc] Using G1
    # WARNING: Unable to get Instrumentation. Dynamic Attach failed. You may add this JAR as -javaagent manually, or supply -Djdk.attach.allowAttachSelf
    [2.540s][info][gc] GC(0) Pause Young (Normal) (G1 Evacuation Pause) 24M->1M(128M) 2.600ms
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           0d 00 00 00 (00001101 00000000 00000000 00000000) (13)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           00 02 00 20 (00000000 00000010 00000000 00100000) (536871424)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    [2.627s][info][gc] GC(1) Pause Young (Normal) (G1 Evacuation Pause) 75M->1M(128M) 2.273ms
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           15 00 00 00 (00010101 00000000 00000000 00000000) (21)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           00 02 00 20 (00000000 00000010 00000000 00100000) (536871424)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    [2.675s][info][gc] GC(2) Pause Young (Normal) (G1 Evacuation Pause) 75M->1M(128M) 2.063ms
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           1d 00 00 00 (00011101 00000000 00000000 00000000) (29)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           00 02 00 20 (00000000 00000010 00000000 00100000) (536871424)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    [2.724s][info][gc] GC(3) Pause Young (Normal) (G1 Evacuation Pause) 75M->1M(128M) 2.068ms
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           25 00 00 00 (00100101 00000000 00000000 00000000) (37)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           00 02 00 20 (00000000 00000010 00000000 00100000) (536871424)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    [2.772s][info][gc] GC(4) Pause Young (Normal) (G1 Evacuation Pause) 75M->1M(128M) 2.212ms
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           2d 00 00 00 (00101101 00000000 00000000 00000000) (45)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           00 02 00 20 (00000000 00000010 00000000 00100000) (536871424)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    [2.821s][info][gc] GC(5) Pause Young (Normal) (G1 Evacuation Pause) 75M->1M(128M) 2.202ms
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           35 00 00 00 (00110101 00000000 00000000 00000000) (53)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           00 02 00 20 (00000000 00000010 00000000 00100000) (536871424)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    [2.869s][info][gc] GC(6) Pause Young (Normal) (G1 Evacuation Pause) 75M->1M(128M) 2.143ms
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           3d 00 00 00 (00111101 00000000 00000000 00000000) (61)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           00 02 00 20 (00000000 00000010 00000000 00100000) (536871424)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    [2.917s][info][gc] GC(7) Pause Young (Normal) (G1 Evacuation Pause) 75M->1M(128M) 2.313ms
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           45 00 00 00 (01000101 00000000 00000000 00000000) (69)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           00 02 00 20 (00000000 00000010 00000000 00100000) (536871424)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    [2.969s][info][gc] GC(8) Pause Young (Normal) (G1 Evacuation Pause) 75M->1M(128M) 2.473ms
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           4d 00 00 00 (01001101 00000000 00000000 00000000) (77)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           00 02 00 20 (00000000 00000010 00000000 00100000) (536871424)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    [3.021s][info][gc] GC(9) Pause Young (Normal) (G1 Evacuation Pause) 75M->1M(128M) 2.283ms
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           55 00 00 00 (01010101 00000000 00000000 00000000) (85)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           00 02 00 20 (00000000 00000010 00000000 00100000) (536871424)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    [3.072s][info][gc] GC(10) Pause Young (Normal) (G1 Evacuation Pause) 75M->1M(128M) 2.648ms
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           5d 00 00 00 (01011101 00000000 00000000 00000000) (93)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           00 02 00 20 (00000000 00000010 00000000 00100000) (536871424)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    [3.122s][info][gc] GC(11) Pause Young (Normal) (G1 Evacuation Pause) 75M->1M(128M) 2.585ms
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           65 00 00 00 (01100101 00000000 00000000 00000000) (101)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           00 02 00 20 (00000000 00000010 00000000 00100000) (536871424)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    [3.173s][info][gc] GC(12) Pause Young (Normal) (G1 Evacuation Pause) 75M->1M(128M) 2.130ms
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           6d 00 00 00 (01101101 00000000 00000000 00000000) (109)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           00 02 00 20 (00000000 00000010 00000000 00100000) (536871424)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    [3.224s][info][gc] GC(13) Pause Young (Normal) (G1 Evacuation Pause) 75M->1M(128M) 2.078ms
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           75 00 00 00 (01110101 00000000 00000000 00000000) (117)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           00 02 00 20 (00000000 00000010 00000000 00100000) (536871424)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    [3.273s][info][gc] GC(14) Pause Young (Normal) (G1 Evacuation Pause) 75M->1M(128M) 2.135ms
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           7d 00 00 00 (01111101 00000000 00000000 00000000) (125)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           00 02 00 20 (00000000 00000010 00000000 00100000) (536871424)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    [3.322s][info][gc] GC(15) Pause Young (Normal) (G1 Evacuation Pause) 75M->1M(128M) 2.467ms
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           7d 00 00 00 (01111101 00000000 00000000 00000000) (125)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           00 02 00 20 (00000000 00000010 00000000 00100000) (536871424)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    [3.404s][info][gc] GC(16) Pause Young (Normal) (G1 Evacuation Pause) 76M->1M(128M) 0.556ms
    [3.485s][info][gc] GC(17) Pause Young (Normal) (G1 Evacuation Pause) 76M->1M(128M) 0.303ms
    [3.566s][info][gc] GC(18) Pause Young (Normal) (G1 Evacuation Pause) 76M->1M(128M) 0.288ms
    [3.647s][info][gc] GC(19) Pause Young (Normal) (G1 Evacuation Pause) 76M->1M(128M) 0.317ms
    [3.727s][info][gc] GC(20) Pause Young (Normal) (G1 Evacuation Pause) 76M->1M(128M) 0.286ms

可以看到，在第 15 次 GC 的时候，对象进入了老年代，内存地址不再随着 Young GC 的进行而变化。  
更对对象头的详细信息，请参考：[Java GC详解 - 1. 最全面的理解Java对象结构 - 对象指针 OOPs][Java GC_ - 1. _Java_ - _ OOPs]

# 动态晋升 #

动态晋升首先根据 TargetSurvivorRatio 指定的比例，乘以 survivor 一个区的大小，得出**目标晋升空间大小**。然后将分代对象大小，**按照分代年龄从小到大相加，直到大于目标晋升空间大小**。之后，将**得出的这个分代年龄以上的对象全部晋升**。

动态修改 Tenuring Threshold，也就是晋升的分代年龄，源代码对应：[`src/hotspot/share/gc/serial/defNewGeneration.cpp`][src_hotspot_share_gc_serial_defNewGeneration.cpp]

void DefNewGeneration::adjust_desired_tenuring_threshold() {
      // 获取 survivor 区大小，有两个 survivor 区，获取当前拷贝目标的那一个的大小
      size_t const survivor_capacity = to()->capacity() / HeapWordSize;
      // 计算 desired_survivor_size，通过 TargetSurvivorRatio
      size_t const desired_survivor_size = (size_t)((((double)survivor_capacity) * TargetSurvivorRatio) / 100);
    
      // 计算目标 Tenuring Threshold
      _tenuring_threshold = age_table()->compute_tenuring_threshold(desired_survivor_size);
    
      //后续都是数据采集统计以及日志，不用看
      if (UsePerfData) {
        GCPolicyCounters* gc_counters = GenCollectedHeap::heap()->counters();
        gc_counters->tenuring_threshold()->set_value(_tenuring_threshold);
        gc_counters->desired_survivor_size()->set_value(desired_survivor_size * oopSize);
      }
    
      age_table()->print_age_table(_tenuring_threshold);
    }

计算目标 Tenuring Threshold 对应源码：[`src/hotspot/share/gc/shared/ageTable.cpp`][src_hotspot_share_gc_shared_ageTable.cpp]

uint AgeTable::compute_tenuring_threshold(size_t desired_survivor_size) {
      uint result;
      //如果是永远直接晋升或者从不晋升，则直接返回结果，不计算
      if (AlwaysTenure || NeverTenure) {
        assert(MaxTenuringThreshold == 0 || MaxTenuringThreshold == markWord::max_age + 1,
               "MaxTenuringThreshold should be 0 or markWord::max_age + 1, but is " UINTX_FORMAT, MaxTenuringThreshold);
        result = MaxTenuringThreshold;
      } else {
        size_t total = 0;
        uint age = 1;
        assert(sizes[0] == 0, "no objects with age zero should be recorded");
        //每个分代对象从分代年龄小加到大，直到大于 desired_survivor_size
        while (age < table_size) {
          total += sizes[age];
          // check if including objects of age 'age' made us pass the desired
          // size, if so 'age' is the new threshold
          if (total > desired_survivor_size) break;
          age++;
        }
        // 获取当前大于 desired_survivor_size 的分代年龄，将这个分代年龄以上的对象全部晋升到老年代
        result = age < MaxTenuringThreshold ? age : MaxTenuringThreshold;
    }

[image]: /images/20221120/546e0cf371f34d208f723cead4bc84a0.png
[JFR _ JVM - TLAB]: https://zhanghaoxin.blog.csdn.net/article/details/111331532
[OpenJDK 11 JVM]: https://juejin.im/post/6873020339752796167
[Java GC_ - 1. _Java_ - _ OOPs]: https://zhuanlan.zhihu.com/p/248682652
[src_hotspot_share_gc_serial_defNewGeneration.cpp]: https://github.com/openjdk/jdk/blob/master/src/hotspot/share/gc/serial/defNewGeneration.cpp
[src_hotspot_share_gc_shared_ageTable.cpp]: https://github.com/openjdk/jdk/blob/master/src/hotspot/share/gc/shared/ageTable.cpp