Java Runtime的使用詳解

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前言

最近做項目框架，需要在框架結束的時候，關閉服務器連接，清除部分框架運行lock文件，這裡就想到瞭shutdownhook，順便學瞭學Runtime的使用

1. shutdownhook

demo示例，證明在程序正常結束的時候會調用，如果kill -9 那肯定就不會調用瞭

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================");
    }
}

正常運行結束，結果如下

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

Process finished with exit code 0

如果暫停，點擊下圖左下角的正方形紅圖標，停止正在運行的應用

結果如下，shutdownhook已執行。

shutdownhook可以處理程序正常結束的時候，刪除文件，關閉連接等

2. exec執行

2.1 常規命令執行

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);
            }
        }
    }
}

結果如下

而正常執行結果

但是這個方法有遠程執行風險，即在瀏覽器端通過這個方法執行特定指令，比如執行rm -rf *，結果就很……

2.2 管道符

但是遇見管道符之後就會失效，什麼辦法解決，sh -c，但是不能直接用，否則獲取到的是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);
            }
        }
    }

結果😓

sh -c的參數要分離，不然runtime會認為是一個參數

2.3源碼分析

跟蹤代碼，使用ProcessImpl來執行指令

    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 環境

 
/**
 * 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. 總結

Runtime用處非常多，偏底層

比如gc調用

加載jar文件

Runtime功能強大，但需要合理利用，很多攻擊是通過Runtime執行的漏洞

但是使用shutdownhook還是很方便的，用來做停止任務的後續處理。

以上為個人經驗，希望能給大傢一個參考，也希望大傢多多支持WalkonNet。

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