Java Runtime的使用详解

2021-12-15 15:01:21 作者：fenglllle

这篇文章主要介绍了Java Runtime的使用详解，具有很好的参考价值，希望对大家有所帮助。如有错误或未考虑完全的地方，望不吝赐教

前言

最近做项目框架，需要在框架结束的时候，关闭服务器连接，清除部分框架运行lock文件，这里就想到了shutdownhook，顺便学了学Runtime的使用

1. shutdownhook

demo示例，证明在程序正常结束的时候会调用，如果kill -9 那肯定就不会调用了

public class ShutdownHookTest { 
    public static void main(String[] args) {
        System.out.println("==============application start================");
 
        Runtime.getRuntime().addShutdownHook(new Thread(()->{
            System.out.println("--------------hook 1----------------");
        }));
        Runtime.getRuntime().addShutdownHook(new Thread(()->{
            System.out.println("--------------hook 2----------------");
        }));
 
        System.out.println("==============application end================");
    }
}

正常运行结束，结果如下

==============application start================
==============application end================
--------------hook 1----------------
--------------hook 2----------------

Process finished with exit code 0

如果暂停，点击下图左下角的正方形红图标，停止正在运行的应用

结果如下，shutdownhook已执行。

shutdownhook可以处理程序正常结束的时候，删除文件，关闭连接等

2. exec执行

2.1 常规命令执行

demo示例如下，比如ls

public class ShutdownHookTest { 
    public static void main(String[] args) throws InterruptedException, IOException {
        Process process = Runtime.getRuntime().exec("ls"); 
        try (InputStream fis = process.getInputStream();
             InputStreamReader isr = new InputStreamReader(fis);
             BufferedReader br = new BufferedReader(isr)) {
            String line;
            while ((line = br.readLine()) != null) {
                System.out.println(line);
            }
        }
    }
}

结果如下

而正常执行结果

但是这个方法有远程执行风险，即在浏览器端通过这个方法执行特定指令，比如执行rm -rf *，结果就很……

2.2 管道符

但是遇见管道符之后就会失效，什么办法解决，sh -c，但是不能直接用，否则获取到的是TTY窗口信息

    public static void main(String[] args) throws IOException {
        Process process = Runtime.getRuntime().exec("sh -c ps aux|grep java"); 
        try (InputStream fis = process.getInputStream();
             InputStreamReader isr = new InputStreamReader(fis);
             BufferedReader br = new BufferedReader(isr)) {
            String line;
            while ((line = br.readLine()) != null) {
                System.out.println(line);
            }
        }
    }

结果😓

sh -c的参数要分离，不然runtime会认为是一个参数

2.3源码分析

跟踪代码，使用ProcessImpl来执行指令

    public Process exec(String[] cmdarray, String[] envp, File dir)
        throws IOException {
        return new ProcessBuilder(cmdarray)
            .environment(envp)
            .directory(dir)
            .start();
    }

ProcessBuilder

// Only for use by ProcessBuilder.start()
    static Process start(String[] cmdarray,
                         java.util.Map<String,String> environment,
                         String dir,
                         ProcessBuilder.Redirect[] redirects,
                         boolean redirectErrorStream)
        throws IOException
    {
        assert cmdarray != null && cmdarray.length > 0;
 
        // Convert arguments to a contiguous block; it's easier to do
        // memory management in Java than in C.
        byte[][] args = new byte[cmdarray.length-1][];
        int size = args.length; // For added NUL bytes
        for (int i = 0; i < args.length; i++) {
            args[i] = cmdarray[i+1].getBytes();
            size += args[i].length;
        }
        byte[] argBlock = new byte[size];
        int i = 0;
        for (byte[] arg : args) {
            System.arraycopy(arg, 0, argBlock, i, arg.length);
            i += arg.length + 1;
            // No need to write NUL bytes explicitly
        }
 
        int[] envc = new int[1];
        byte[] envBlock = ProcessEnvironment.toEnvironmentBlock(environment, envc); 
        int[] std_fds; 
        FileInputStream  f0 = null;
        FileOutputStream f1 = null;
        FileOutputStream f2 = null;
 
        try {
            if (redirects == null) {
                std_fds = new int[] { -1, -1, -1 };
            } else {
                std_fds = new int[3];
 
                if (redirects[0] == Redirect.PIPE)
                    std_fds[0] = -1;
                else if (redirects[0] == Redirect.INHERIT)
                    std_fds[0] = 0;
                else {
                    f0 = new FileInputStream(redirects[0].file());
                    std_fds[0] = fdAccess.get(f0.getFD());
                }
 
                if (redirects[1] == Redirect.PIPE)
                    std_fds[1] = -1;
                else if (redirects[1] == Redirect.INHERIT)
                    std_fds[1] = 1;
                else {
                    f1 = new FileOutputStream(redirects[1].file(),
                                              redirects[1].append());
                    std_fds[1] = fdAccess.get(f1.getFD());
                }
 
                if (redirects[2] == Redirect.PIPE)
                    std_fds[2] = -1;
                else if (redirects[2] == Redirect.INHERIT)
                    std_fds[2] = 2;
                else {
                    f2 = new FileOutputStream(redirects[2].file(),
                                              redirects[2].append());
                    std_fds[2] = fdAccess.get(f2.getFD());
                }
            }
 
        return new UNIXProcess
            (toCString(cmdarray[0]),
             argBlock, args.length,
             envBlock, envc[0],
             toCString(dir),
                 std_fds,
             redirectErrorStream);
        } finally {
            // In theory, close() can throw IOException
            // (although it is rather unlikely to happen here)
            try { if (f0 != null) f0.close(); }
            finally {
                try { if (f1 != null) f1.close(); }
                finally { if (f2 != null) f2.close(); }
            }
        }
    }

new UNIXProcess 环境

 
/**
 * java.lang.Process subclass in the UNIX environment.
 *
 * @author Mario Wolczko and Ross Knippel.
 * @author Konstantin Kladko (ported to Linux and Bsd)
 * @author Martin Buchholz
 * @author Volker Simonis (ported to AIX)
 */
final class UNIXProcess extends Process {

3. 总结

Runtime用处非常多，偏底层

比如gc调用

加载jar文件

Runtime功能强大，但需要合理利用，很多攻击是通过Runtime执行的漏洞

但是使用shutdownhook还是很方便的，用来做停止任务的后续处理。

以上为个人经验，希望能给大家一个参考，也希望大家多多支持脚本之家。