java - JVM 命令启动选项重复

Question

我发现我们正在使用的旧版软件的启动属性格式错误，因此它接收这两个不相等的 xmx 作为属性：

java -jar myapp.jar -Xmx128m -Xmx512m 

我无法访问启动器源代码（无法修改它），所以我问，这些参数重复有什么影响？我可以这样离开吗，还是我应该担心？哪一个会被应用？

使用的 JVM 是 JRE 6 update 18

Answer 1

In general, it's usually the latter option that gets used if a tool doesn't reject a duplicate, but you can't count on that unless the tool documents it.

Your best bet is to see what happens with your specific JVM, via Runtime's totalMemory and maxMemory:

public class HeapSize {
    public static final void main(String[] args) {
        Runtime rt = Runtime.getRuntime();
        System.out.println("Total currently: " + rt.totalMemory());
        System.out.println("Max:             " + rt.maxMemory());
        System.exit(0);
    }
}

On my JVM (Sun/Oracle 1.6.0_26-b03 under Linux), the latter option takes effect:

$ java -Xmx16m HeapSize
Total currently: 16121856
Max:             16121856
$ java -Xmx32m HeapSize
Total currently: 32178176
Max:             32178176
$ java -Xmx16m -Xmx32m HeapSize
Total currently: 32178176
Max:             32178176
$ java -Xmx16m -Xmx32m -Xmx128m HeapSize
Total currently: 59113472
Max:             119341056

Answer 2

我的理解是它将使用最后一个设置。