netty maxDirectMemory 配置

初始化位置

先判断 vmName 不是 ibm j9 并且不是 eclipse openj9，通过反射获取sun.misc.VM，当前版本异常，走后面逻辑；
通过正则获取 -XX:MaxDirectMemorySize 配置，如果有，则使用，没有走后面逻辑，maxDirectMemory = Runtime.getRuntime().maxMemory();；

// io.netty.util.internal.PlatformDependent#maxDirectMemory0
    private static long maxDirectMemory0() {
        long maxDirectMemory = 0;

        ClassLoader systemClassLoader = null;
        try {
            systemClassLoader = getSystemClassLoader();

            // When using IBM J9 / Eclipse OpenJ9 we should not use VM.maxDirectMemory() as it not reflects the
            // correct value.
            // See:
            //  - https://github.com/netty/netty/issues/7654
            String vmName = SystemPropertyUtil.get("java.vm.name", "").toLowerCase();
            if (!vmName.startsWith("ibm j9") &&
                    // https://github.com/eclipse/openj9/blob/openj9-0.8.0/runtime/include/vendor_version.h#L53
                    !vmName.startsWith("eclipse openj9")) {
                // Try to get from sun.misc.VM.maxDirectMemory() which should be most accurate.
                Class<?> vmClass = Class.forName("sun.misc.VM", true, systemClassLoader);
                Method m = vmClass.getDeclaredMethod("maxDirectMemory");
                maxDirectMemory = ((Number) m.invoke(null)).longValue();
            }
        } catch (Throwable ignored) {
            // Ignore
        }

        if (maxDirectMemory > 0) {
            return maxDirectMemory;
        }

        try {
            // Now try to get the JVM option (-XX:MaxDirectMemorySize) and parse it.
            // Note that we are using reflection because Android doesn't have these classes.
            Class<?> mgmtFactoryClass = Class.forName(
                    "java.lang.management.ManagementFactory", true, systemClassLoader);
            Class<?> runtimeClass = Class.forName(
                    "java.lang.management.RuntimeMXBean", true, systemClassLoader);

            Object runtime = mgmtFactoryClass.getDeclaredMethod("getRuntimeMXBean").invoke(null);

            @SuppressWarnings("unchecked")
            List<String> vmArgs = (List<String>) runtimeClass.getDeclaredMethod("getInputArguments").invoke(runtime);
            for (int i = vmArgs.size() - 1; i >= 0; i --) {
                Matcher m = MAX_DIRECT_MEMORY_SIZE_ARG_PATTERN.matcher(vmArgs.get(i));
                if (!m.matches()) {
                    continue;
                }

                maxDirectMemory = Long.parseLong(m.group(1));
                switch (m.group(2).charAt(0)) {
                    case 'k': case 'K':
                        maxDirectMemory *= 1024;
                        break;
                    case 'm': case 'M':
                        maxDirectMemory *= 1024 * 1024;
                        break;
                    case 'g': case 'G':
                        maxDirectMemory *= 1024 * 1024 * 1024;
                        break;
                    default:
                        break;
                }
                break;
            }
        } catch (Throwable ignored) {
            // Ignore
        }

        if (maxDirectMemory <= 0) {
            maxDirectMemory = Runtime.getRuntime().maxMemory();
            logger.debug("maxDirectMemory: {} bytes (maybe)", maxDirectMemory);
        } else {
            logger.debug("maxDirectMemory: {} bytes", maxDirectMemory);
        }

        return maxDirectMemory;
    }