校验一个字符串是否是合法的JSON格式

private CharacterIterator it;
private char c;
private int col;

public JsonValidator() {
}

/**
* 验证一个字符串是否是合法的JSON串
*
* @param input 要验证的字符串
* @return true-合法 ，false-非法
*/
public boolean validate(String input) {
input = input.trim();
boolean ret = valid(input);
return ret;
}

private boolean valid(String input) {
if ("".equals(input)) return true;

boolean ret = true;
it = new StringCharacterIterator(input);
c = it.first();
col = 1;
if (!value()) {
ret = error("value", 1);
} else {
skipWhiteSpace();
if (c != CharacterIterator.DONE) {
ret = error("end", col);
}
}

return ret;
}

private boolean value() {
return literal("true") || literal("false") || literal("null") || string() || number() || object() || array();
}

private boolean literal(String text) {
CharacterIterator ci = new StringCharacterIterator(text);
char t = ci.first();
if (c != t) return false;

int start = col;
boolean ret = true;
for (t = ci.next(); t != CharacterIterator.DONE; t = ci.next()) {
if (t != nextCharacter()) {
ret = false;
break;
}
}
nextCharacter();
if (!ret) error("literal " + text, start);
return ret;
}

private boolean array() {
return aggregate('[', ']', false);
}

private boolean object() {
return aggregate('{', '}', true);
}

private boolean aggregate(char entryCharacter, char exitCharacter, boolean prefix) {
if (c != entryCharacter) return false;
nextCharacter();
skipWhiteSpace();
if (c == exitCharacter) {
nextCharacter();
return true;
}

for (; ; ) {
if (prefix) {
int start = col;
if (!string()) return error("string", start);
skipWhiteSpace();
if (c != ':') return error("colon", col);
nextCharacter();
skipWhiteSpace();
}
if (value()) {
skipWhiteSpace();
if (c == ',') {
nextCharacter();
} else if (c == exitCharacter) {
break;
} else {
return error("comma or " + exitCharacter, col);
}
} else {
return error("value", col);
}
skipWhiteSpace();
}

nextCharacter();
return true;
}

private boolean number() {
if (!Character.isDigit(c) && c != '-') return false;
int start = col;
if (c == '-') nextCharacter();
if (c == '0') {
nextCharacter();
} else if (Character.isDigit(c)) {
while (Character.isDigit(c))
nextCharacter();
} else {
return error("number", start);
}
if (c == '.') {
nextCharacter();
if (Character.isDigit(c)) {
while (Character.isDigit(c))
nextCharacter();
} else {
return error("number", start);
}
}
if (c == 'e' || c == 'E') {
nextCharacter();
if (c == '+' || c == '-') {
nextCharacter();
}
if (Character.isDigit(c)) {
while (Character.isDigit(c))
nextCharacter();
} else {
return error("number", start);
}
}
return true;
}

private boolean string() {
if (c != '"') return false;

int start = col;
boolean escaped = false;
for (nextCharacter(); c != CharacterIterator.DONE; nextCharacter()) {
if (!escaped && c == '\\') {
escaped = true;
} else if (escaped) {
if (!escape()) {
return false;
}
escaped = false;
} else if (c == '"') {
nextCharacter();
return true;
}
}
return error("quoted string", start);
}

private boolean escape() {
int start = col - 1;
if (" \\\"/bfnrtu".indexOf(c) < 0) {
return error("escape sequence  \\\",\\\\,\\/,\\b,\\f,\\n,\\r,\\t  or  \\uxxxx ", start);
}
if (c == 'u') {
if (!ishex(nextCharacter()) || !ishex(nextCharacter()) || !ishex(nextCharacter())
|| !ishex(nextCharacter())) {
return error("unicode escape sequence  \\uxxxx ", start);
}
}
return true;
}

private boolean ishex(char d) {
return "0123456789abcdefABCDEF".indexOf(c) >= 0;
}

private char nextCharacter() {
c = it.next();
++col;
return c;
}

private void skipWhiteSpace() {
while (Character.isWhitespace(c)) {
nextCharacter();
}
}

private boolean error(String type, int col) {
System.out.printf("type: %s, col: %s%s", type, col, System.getProperty("line.separator"));
return false;
}