try (RandomByteInputStream is = new RandomByteInputStream(1_000_000)) {
    for (int b = is.read(); b >= 0; b = is.read()) {
        System.out.println(b);
    }
}

// --------------

public static final class RandomByteInputStream extends java.io.InputStream {

    private long size;
    private long eoln;
    private long pos;
    private final static int EOLN_POS_MAX = 4096;
    private final static char EOLN_CHAR = '\n';
    private final static java.util.Random RND = new java.util.Random();

    public RandomByteInputStream(long size) {
        this.size = size;
        this.eoln = -1;
        this.pos = 0;
    }

    @Override
    public int read() throws IOException {
        int nextByte = RND.nextInt() & 0x7F;
        if (this.eoln == -1) {
            this.eoln = RND.nextInt(EOLN_POS_MAX) + 1;
        }
        if (this.pos >= this.eoln) {
            this.eoln = -1;
            nextByte = EOLN_CHAR;
        } 
        return this.pos++ < this.size ? nextByte : -1;
    }
}

RandomStringGenerator rsg = new RandomStringGenerator(20, false);
for (int i = 0; i < 100; i++) {
    System.out.println(rsg);
}

public static final class RandomStringGenerator {
    
    private final int length;    // максимальная длина строки
    private final boolean fixed; // true - всегда фиксированной длины
    
    private java.util.Random RND = new java.util.Random();

    public RandomStringGenerator(int length, boolean fixed) {
        this.length = length;
        this.fixed = fixed;
    }
    
    private int getLength() {
        return this.fixed ? this.length : RND.nextInt(this.length) + 1;
    }
    
    @Override
    public String toString() {
        return org.apache.commons.lang.RandomStringUtils.random(this.getLength(), true, true);
    }
}

<dependency>
    <groupId>commons-lang</groupId>
    <artifactId>commons-lang</artifactId>
    <version>2.6</version>
</dependency>