如何转储来自 JVM 堆老年代的 Java 对象？

有没有工具可以转储旧代 JVM 堆？

换句话说，我如何判断一个对象是来自年轻代还是老年代？

如果您运行 Oracle JDK 或 OpenJDK，则可以使用以下命令执行此操作热点可服务性代理 http://openjdk.java.net/groups/hotspot/docs/Serviceability.html#bsa sa-jdi.jar。它可以发现老年代的边界。下面是一个收集 OldGen 边界内的对象之间的堆直方图的示例。

还可以从 Java 进程中找到老年代的地址，请参阅相关问题 https://stackoverflow.com/questions/30599986/how-to-find-out-where-exact-young-old-gen-is-located-in-memory.

import sun.jvm.hotspot.gc_implementation.parallelScavenge.ParallelScavengeHeap;
import sun.jvm.hotspot.gc_interface.CollectedHeap;
import sun.jvm.hotspot.memory.GenCollectedHeap;
import sun.jvm.hotspot.memory.MemRegion;
import sun.jvm.hotspot.oops.ObjectHistogram;
import sun.jvm.hotspot.oops.Oop;
import sun.jvm.hotspot.runtime.VM;
import sun.jvm.hotspot.tools.Tool;

public class OldGen extends Tool {

    public static void main(String[] args) {
        new OldGen().execute(args);
    }

    @Override
    public void run() {
        MemRegion oldRegion = getOldRegion(VM.getVM().getUniverse().heap());

        ObjectHistogram histogram = new ObjectHistogram() {
            @Override
            public boolean doObj(Oop obj) {
                return oldRegion.contains(obj.getHandle()) && super.doObj(obj);
            }
        };

        VM.getVM().getObjectHeap().iterate(histogram);
        histogram.print();
    }

    private MemRegion getOldRegion(CollectedHeap heap) {
        if (heap instanceof ParallelScavengeHeap) {
            return ((ParallelScavengeHeap) heap).oldGen().objectSpace().usedRegion();
        } else if (heap instanceof GenCollectedHeap) {
            return ((GenCollectedHeap) heap).getGen(1).usedRegion();
        } else {
            throw new UnsupportedOperationException(heap.kind() + " is not supported");
        }
    }
}

UPDATE

用于 JDK 11/17 和 G1GC 的类似工具：

// Add the following JVM options to run
// --add-modules jdk.hotspot.agent
// --add-exports jdk.hotspot.agent/sun.jvm.hotspot.debugger=ALL-UNNAMED
// --add-exports jdk.hotspot.agent/sun.jvm.hotspot.gc.g1=ALL-UNNAMED
// --add-exports jdk.hotspot.agent/sun.jvm.hotspot.oops=ALL-UNNAMED
// --add-exports jdk.hotspot.agent/sun.jvm.hotspot.runtime=ALL-UNNAMED
// --add-exports jdk.hotspot.agent/sun.jvm.hotspot.tools=ALL-UNNAMED
// --add-exports jdk.hotspot.agent/sun.jvm.hotspot.types=ALL-UNNAMED
// --add-exports jdk.hotspot.agent/sun.jvm.hotspot.memory=ALL-UNNAMED

import sun.jvm.hotspot.debugger.Address;
import sun.jvm.hotspot.debugger.AddressException;
import sun.jvm.hotspot.debugger.OopHandle;
import sun.jvm.hotspot.gc.g1.G1CollectedHeap;
import sun.jvm.hotspot.gc.g1.HeapRegion;
import sun.jvm.hotspot.oops.Klass;
import sun.jvm.hotspot.oops.ObjectHeap;
import sun.jvm.hotspot.oops.Oop;
import sun.jvm.hotspot.oops.UnknownOopException;
import sun.jvm.hotspot.runtime.VM;
import sun.jvm.hotspot.tools.Tool;
import sun.jvm.hotspot.types.WrongTypeException;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.concurrent.atomic.AtomicLong;

public class OldGenHistogram extends Tool {
    final Map<Klass, AtomicLong> histogram = new HashMap<>();

    @Override
    public void run() {
        G1CollectedHeap g1Heap = (G1CollectedHeap) VM.getVM().getUniverse().heap();
        for (Iterator<HeapRegion> it = g1Heap.hrm().heapRegionIterator(); it.hasNext(); ) {
            HeapRegion hr = it.next();
            if (hr.isOld()) {
                iterate(hr);
            }
        }

        histogram.entrySet().stream()
                .sorted((e1, e2) -> Long.compare(e2.getValue().longValue(), e1.getValue().longValue()))
                .forEach(e -> {
                    System.out.print(e.getValue() + "  ");
                    e.getKey().printValueOn(System.out);
                    System.out.println();
                });
    }

    private void iterate(HeapRegion region) {
        ObjectHeap heap = VM.getVM().getObjectHeap();
        Address bottom = region.bottom();
        Address top = region.top();

        try {
            OopHandle handle = bottom.addOffsetToAsOopHandle(0);
            while (handle.lessThan(top)) {
                Oop obj = heap.newOop(handle);
                long size = obj.getObjectSize();
                histogram.computeIfAbsent(obj.getKlass(), k -> new AtomicLong())
                        .addAndGet(size);
                handle = handle.addOffsetToAsOopHandle(size);
            }
        } catch (AddressException | UnknownOopException | WrongTypeException e) {
            // skip
        }
    }

    public static void main(String[] args) {
        new OldGenHistogram().execute(args);
    }
}