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？
Posted 干货满满张哈希
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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。
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
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
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
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
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
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;
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