如何將Java MistageDigest用於哈希（MD5，SHA-256）？

要使用Java生成哈希值，可通過MessageDigest類實現。 1. 獲取指定算法的實例，如MD5或SHA-256；2. 調用.update()方法傳入待加密數據；3. 調用.digest()方法獲取哈希字節數組；4. 將字節數組轉換為十六進製字符串以便讀取；對於大文件等輸入，應分塊讀取並多次調用.update()；推薦使用SHA-256而非MD5或SHA-1以確保安全性。

You can use Java's MessageDigest class to generate hash values like MD5, SHA-256, and others without relying on external libraries. It's straightforward once you get the steps right.

Basic Usage of MessageDigest

The core idea is to get an instance of the desired algorithm (like MD5 or SHA-256), update it with the data you want to hash, and then compute the digest. Here's a quick breakdown:

1. Use MessageDigest.getInstance("MD5") or MessageDigest.getInstance("SHA-256")
2. Call .update() with your input bytes
3. Call .digest() to get the hashed byte array

For example:

 MessageDigest md = MessageDigest.getInstance("SHA-256");
md.update("hello".getBytes());
byte[] hash = md.digest();

That gives you the raw hash value in a byte array. From here, you'll usually convert that into a hex string for readability.

Converting Byte Array to Hex String

The result from .digest() is a byte array — not very readable. You need to convert it to hexadecimal format so it can be stored or compared easily.

A common way to do this is by looping through each byte and formatting it as two hex digits:

 StringBuilder sb = new StringBuilder();
for (byte b : hash) {
    sb.append(String.format(" x", b));
}
String hexHash = sb.toString();

Make sure to use x instead of %x — otherwise, negative bytes will look weird due to sign extension. This part trips up many beginners.

Handling Different Inputs

You might not always hash plain strings. Sometimes it's files, streams, or other binary data.

When hashing a file, for example, read it in chunks and call .update() multiple times before calling .digest() :

 MessageDigest md = MessageDigest.getInstance("SHA-256");

try (InputStream is = new FileInputStream("file.txt")) {
    byte[] buffer = new byte[8192];
    int bytesRead;
    while ((bytesRead = is.read(buffer)) != -1) {
        md.update(buffer, 0, bytesRead);
    }
}
byte[] hash = md.digest();

This approach avoids loading the whole file into memory at once and works well even for large files.

Choosing the Right Algorithm

MD5 and SHA-256 are just two options. Java supports several algorithms out of the box, including:

• SHA-1
• SHA-224
• SHA-256
• SHA-384
• SHA-512
• MD2, MD5

Keep in mind: MD5 is considered cryptographically broken and should not be used for security-sensitive applications. SHA-256 (part of the SHA-2 family) is much safer and widely recommended.

If you're working in a security context, avoid MD5 and SHA-1 unless you're using them only for checksums or non-security purposes.

基本上就這些。

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