A question about ManagementFactory.getGarbageCollectorMXBeans

Question

i want to count the number of fullgc, this is my code

Thread t = new Thread(new Runnable() {
    @Override
    public void run() {
        long fullGcCount = 0;
        for(GarbageCollectorMXBean bean: ManagementFactory.getGarbageCollectorMXBeans()) {
            System.out.println(bean.getName());
            if(bean.getName().equals("ConcurrentMarkSweep")) {
                logger.info("init fullgc count:{}", bean.getCollectionCount());
                while (true) {
                    long temGcCount = bean.getCollectionCount();
                    if(temGcCount == fullGcCount) {
                        continue;
                    }else {
                        System.out.println(String.format("before the FullGC, sum of fullgc:%s, add FullGC count:%s", fullGcCount, temGcCount - fullGcCount));
                        fullGcCount = temGcCount;
                    }
                }
            }
        }
    }
});
t.start();

i run jmap -histo:live 2 times. this is the first gc log：

2023-01-10T19:18:03.496+0800: 80595.099: [Full GC (Heap Inspection Initiated GC) 80595.099: [CMS[YG occupancy: 168131 K (188736 K)]80595.174: [weak refs processing, 0.0015483 secs]80595.176: [class unloading, 0.0253856 secs]80595.201: [scrub symbol table, 0.0237105 secs]80595.225: [scrub string table, 0.0020402 secs]: 112558K->83931K(838912K), 0.2212898 secs] 280689K->252063K(1027648K), [Metaspace: 117424K->117424K(1155072K)], 0.2218481 secs] [Times: user=0.39 sys=0.04, real=0.22 secs] 

this is the second gc log：

2023-01-10T19:18:29.536+0800: 80621.139: [Full GC (Heap Inspection Initiated GC) 80621.139: [CMS: 83931K->85173K(838912K), 0.3054333 secs] 108263K->85173K(1027648K), [Metaspace: 117022K->117022K(1155072K)], 0.3058419 secs] [Times: user=0.29 sys=0.01, real=0.31 secs] 

It looks like one at a time. But my program thinks the first one is 2 fullgc.

......
before the FullGC, sum of fullgc:1, add FullGC count:2
before the FullGC, sum of fullgc:3, add FullGC count:1
......

but i do not know why. please help!

i use the google but not find the answer.

Answer 1

the cmsgc of 'jmap -histo:live' is foreground. in this model, the gccount will add 1 when 'Initial Mark', 'CMS remark' and 'Sweep'. According to the log, the first gc trigger 'CMS remark' and 'sweep', the second gc trigger 'sweep'.