前言

在默认情况下，System.gc()会显示直接触发Full GC，同时对老年代和新生代进行回收。而一般情况下，垃圾回收应该是自动进行的，无需手工触发，否则就太麻烦了。

示例

public class TestGC {public static void main(String[] args) {new TestGC();System.gc();System.runFinalization();}
}

如果在idea查看日志输出，需要在Run–>Edit Configuration–>VM Options中设置

-XX:+PrintGCDetails

打印结果

[GC (System.gc()) [PSYoungGen: 8489K->1707K(38400K)] 8489K->1715K(125952K), 0.0070544 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] 
[Full GC (System.gc()) [PSYoungGen: 1707K->0K(38400K)] [ParOldGen: 8K->1568K(87552K)] 1715K->1568K(125952K), [Metaspace: 3357K->3357K(1056768K)], 0.0228661 secs] [Times: user=0.03 sys=0.00, real=0.02 secs] 
HeapPSYoungGen      total 38400K, used 998K [0x00000000d5f00000, 0x00000000d8980000, 0x0000000100000000)eden space 33280K, 3% used [0x00000000d5f00000,0x00000000d5ff9b20,0x00000000d7f80000)from space 5120K, 0% used [0x00000000d7f80000,0x00000000d7f80000,0x00000000d8480000)to   space 5120K, 0% used [0x00000000d8480000,0x00000000d8480000,0x00000000d8980000)ParOldGen       total 87552K, used 1568K [0x0000000081c00000, 0x0000000087180000, 0x00000000d5f00000)object space 87552K, 1% used [0x0000000081c00000,0x0000000081d883f0,0x0000000087180000)Metaspace       used 3369K, capacity 4496K, committed 4864K, reserved 1056768Kclass space    used 366K, capacity 388K, committed 512K, reserved 1048576K

在这个例子中，一个新的对象被创建，由于它没有被使用，所以该对象迅速地变为不可达(Unreachable)。
GC开头的说明此次垃圾回收为Minor GC，而Full GC开头的说明此次垃圾回收为stop-the-world的类型。
PSYoungGen表示年轻代，ParOldGen表示老年代。
方括号内的8489K->1707K(38400K)表示GC前该内存区域已使用容量->GC后该内存区域已使用容量（该内存区域总容量）。
方括号外的8489K->1715K(125952K)表示GC前Java堆已使用容量->GC后Java堆已使用容量（Java堆总容量）。
0.0070544 secs表示该内存区域本次GC所占用的时间，单位是秒。有的收集器会给出更具体的时间数据，如[Times: user=0.00 sys=0.00, real=0.01 secs]，这里的user、sys、real与Linux的time命令所输出的时间含义一致，分别代表用户态消耗的CPU时间、内核态消耗的CPU时间、操作从开始到结束所经过的墙钟时间。CPU时间和墙钟时间的区别是，墙钟时间包括各种非运算的等待耗时，如等待磁盘I/O、等待线程阻塞，而CPU时间不包括这些耗时，但当系统有多CPU或者多核的话，多线程操作会叠加这些CPU时间，所以若看到user或sys时间超过real时间也是完全正常的。
eden、from、to表示年轻代中各区域的划分。
Metaspace表示元空间的大小，这里用的是JDK1.8，如果是JDK1.7则没有该区域。

默认情况下，即使System.gc()生效，他会使用传统的Full GC方式回收整个堆内存垃圾，而会忽略选项中的USeG1GC，比如我们设置-XX:+PrintGCDetails -XX:+UseG1GC,运行代码，输出结果如下：

[GC pause (G1 Evacuation Pause) (young), 0.0065937 secs][Parallel Time: 1.2 ms, GC Workers: 4][GC Worker Start (ms): Min: 262.2, Avg: 262.5, Max: 263.4, Diff: 1.2][Ext Root Scanning (ms): Min: 0.0, Avg: 0.2, Max: 0.3, Diff: 0.3, Sum: 0.7][Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][Processed Buffers: Min: 0, Avg: 0.0, Max: 0, Diff: 0, Sum: 0][Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][Object Copy (ms): Min: 0.0, Avg: 0.7, Max: 0.9, Diff: 0.9, Sum: 2.7][Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.2][Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 4][GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][GC Worker Total (ms): Min: 0.0, Avg: 0.9, Max: 1.2, Diff: 1.2, Sum: 3.6][GC Worker End (ms): Min: 263.4, Avg: 263.4, Max: 263.4, Diff: 0.0][Code Root Fixup: 0.0 ms][Code Root Purge: 0.0 ms][Clear CT: 0.0 ms][Other: 5.4 ms][Choose CSet: 0.0 ms][Ref Proc: 5.3 ms][Ref Enq: 0.0 ms][Redirty Cards: 0.0 ms][Humongous Register: 0.0 ms][Humongous Reclaim: 0.0 ms][Free CSet: 0.0 ms][Eden: 6144.0K(6144.0K)->0.0B(12.0M) Survivors: 0.0B->1024.0K Heap: 7086.4K(128.0M)->1579.0K(128.0M)][Times: user=0.00 sys=0.00, real=0.01 secs] 
[Full GC (System.gc())  2078K->1176K(8192K), 0.0057042 secs][Eden: 1024.0K(12.0M)->0.0B(3072.0K) Survivors: 1024.0K->0.0B Heap: 2078.9K(128.0M)->1176.6K(8192.0K)], [Metaspace: 3358K->3358K(1056768K)][Times: user=0.00 sys=0.00, real=0.00 secs] 
Heapgarbage-first heap   total 8192K, used 1176K [0x0000000081c00000, 0x0000000081d00040, 0x0000000100000000)region size 1024K, 1 young (1024K), 0 survivors (0K)Metaspace       used 3368K, capacity 4496K, committed 4864K, reserved 1056768Kclass space    used 366K, capacity 388K, committed 512K, reserved 1048576K

可以看出G1 GC并没有并发执行。打开虚拟机选项 -XX:+ExplicitGCInvokesConcurrent后可改变这种默认行为

[GC pause (System.gc()) (young) (initial-mark), 0.0017516 secs][Parallel Time: 1.4 ms, GC Workers: 4][GC Worker Start (ms): Min: 254.2, Avg: 254.3, Max: 254.5, Diff: 0.3][Ext Root Scanning (ms): Min: 0.0, Avg: 0.4, Max: 1.4, Diff: 1.3, Sum: 1.8][Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][Processed Buffers: Min: 0, Avg: 0.0, Max: 0, Diff: 0, Sum: 0][Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][Object Copy (ms): Min: 0.0, Avg: 0.8, Max: 1.1, Diff: 1.0, Sum: 3.0][Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1][Termination Attempts: Min: 1, Avg: 28.3, Max: 47, Diff: 46, Sum: 113][GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1][GC Worker Total (ms): Min: 1.1, Avg: 1.3, Max: 1.4, Diff: 0.3, Sum: 5.0][GC Worker End (ms): Min: 255.6, Avg: 255.6, Max: 255.6, Diff: 0.0][Code Root Fixup: 0.0 ms][Code Root Purge: 0.0 ms][Clear CT: 0.0 ms][Other: 0.3 ms][Choose CSet: 0.0 ms][Ref Proc: 0.2 ms][Ref Enq: 0.0 ms][Redirty Cards: 0.0 ms][Humongous Register: 0.0 ms][Humongous Reclaim: 0.0 ms][Free CSet: 0.0 ms][Eden: 7168.0K(14.0M)->0.0B(12.0M) Survivors: 0.0B->2048.0K Heap: 7676.5K(128.0M)->1665.6K(128.0M)][Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC concurrent-root-region-scan-start]
[GC concurrent-root-region-scan-end, 0.0007669 secs]
[GC concurrent-mark-start]
[GC concurrent-mark-end, 0.0001007 secs]
[GC remark [Finalize Marking, 0.0000437 secs] [GC ref-proc, 0.0000645 secs] [Unloading, 0.0004621 secs], 0.0006229 secs][Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC cleanup 1665K->1665K(128M), 0.0001751 secs][Times: user=0.00 sys=0.00, real=0.00 secs] 
Heapgarbage-first heap   total 131072K, used 1665K [0x0000000081c00000, 0x0000000081d00400, 0x0000000100000000)region size 1024K, 3 young (3072K), 2 survivors (2048K)Metaspace       used 3373K, capacity 4496K, committed 4864K, reserved 1056768Kclass space    used 366K, capacity 388K, committed 512K, reserved 1048576K

调用System.gc()也仅仅是一个请求垃圾回收的建议，JVM接收这个消息后并不是立即做垃圾回收，而是对几个垃圾回收算法做加权，使垃圾回收操作容易发生或者提早发生，或回收较多而已。