写一个java函数public static String readFileContentString folderPath传入一个apk的路径要求读取根目录下META-INF下的以RSA为后缀的文件为File file传入 return readBinaryFilefile;

以下是实现该函数的Java代码：

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;

public class Main {
    public static void main(String[] args) {
        String folderPath = "path/to/apk/folder";
        String content = readFileContent(folderPath);
        System.out.println(content);
    }

    public static String readFileContent(String folderPath) {
        File folder = new File(folderPath);
        File[] files = folder.listFiles();

        if (files != null) {
            for (File file : files) {
                if (file.isFile() && file.getName().endsWith(".RSA")) {
                    return readBinaryFile(file);
                }
            }
        }

        return "";
    }

    public static String readBinaryFile(File file) {
        try (FileInputStream fis = new FileInputStream(file)) {
            byte[] data = new byte[(int) file.length()];
            fis.read(data);
            return new String(data);
        } catch (IOException e) {
            e.printStackTrace();
        }

        return "";
    }
}

请注意，上述代码中的"path/to/apk/folder"需要替换为实际的APK文件夹路径