How to Safely Validate Command-Line Arguments in Spring Boot Applications

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Validate every argument before opening files, contacting services, or launching subprocesses. For configuration-like options, bind a dedicated prefix such as cli to a typed @ConfigurationProperties class and apply Bean Validation. For positional arguments, subcommands, duplicate-option rules, or strict CLI syntax, parse ApplicationArguments explicitly. Neither approach makes operating-system command execution safe by itself.

How Spring Boot interprets command-line arguments

By default, Spring Boot adds arguments beginning with -- to the application Environment as command-line properties. These properties take precedence over file-based configuration, so an invocation such as java -jar app.jar --server.port=9090 can override a configured server port. See Spring Boot externalized configuration.

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ApplicationArguments provides a structured view of the original arguments:

• getSourceArgs() returns the raw argument array.
• getOptionNames() returns option names, such as foo for --foo=bar.
• containsOption("foo") checks whether an option was supplied.
• getOptionValues("foo") returns null when absent, an empty list for a flag such as --foo, and one or more values for valued or repeated options.
• getNonOptionArgs() returns positional arguments.

These distinctions matter: an absent value, a bare flag, and --name= are not the same input. Define the accepted spelling, whether repetition is allowed, how flags work, and what -- means for your CLI. See the ApplicationArguments API.

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Choose a parsing model that matches the interface

Spring Boot documents @ConfigurationProperties as the type-safe, structured alternative to scattered @Value injection. For a CLI with a formal grammar, treat arguments as a CLI interface rather than assuming configuration binding supplies parsing, help, or positional-argument rules.

Bind and validate configuration-style options

Reserve an application-specific prefix and keep its contract in one typed class. The example below requires an operation, constrains its spelling, bounds a count, and limits an input string’s length:

package com.example.demo;

import jakarta.validation.constraints.Max;
import jakarta.validation.constraints.Min;
import jakarta.validation.constraints.NotBlank;
import jakarta.validation.constraints.NotNull;
import jakarta.validation.constraints.Pattern;
import jakarta.validation.constraints.Size;

import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.validation.annotation.Validated;

@ConfigurationProperties(prefix = "cli", ignoreUnknownFields = false)
@Validated
public record CliOptions(
        @NotBlank
        @Pattern(regexp = "[a-z0-9-]{1,32}")
        String operation,

        @NotNull
        @Min(1)
        @Max(1000)
        Integer count,

        @Size(max = 4096)
        String input) {
}

Register configuration-properties scanning on the application:

@SpringBootApplication
@ConfigurationPropertiesScan
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

Add the validation starter so Bean Validation is available:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-validation</artifactId>
</dependency>

Then invoke the application with options under the prefix:

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java -jar app.jar \
  --cli.operation=import \
  --cli.count=10 \
  --cli.input=/work/data.csv

Spring Boot validates a configuration-properties type annotated with @Validated. Nested objects need @Valid for cascaded validation. Unknown properties are ignored by default, so ignoreUnknownFields=false makes undeclared properties under this bound prefix fail binding. Invalid type conversion is not ignored by default. These settings apply to the bound properties object, not every option supplied to the process. See the externalized configuration reference and ConfigurationProperties API.

The example uses jakarta.validation. Spring Boot 2 applications generally use javax.validation; check the imports against the application’s Boot major version. See the Spring Boot 3 migration guide.

Reject options outside the declared CLI contract

Strict binding of cli.* does not reject an unrelated option such as --clii.count=10. Inspect option names and compare them with the complete set the application intends to accept:

private static final Set<String> ALLOWED = Set.of(
        "cli.operation",
        "cli.count",
        "cli.input",
        "help"
);

static void rejectUnknown(ApplicationArguments args) {
    List<String> unknown = args.getOptionNames().stream()
            .filter(name -> !ALLOWED.contains(name))
            .sorted()
            .toList();

    if (!unknown.isEmpty()) {
        throw new IllegalArgumentException(
                "Unknown option(s): " + unknown);
    }
}

Decide explicitly whether Spring options such as --debug belong to the public contract. If an option may repeat, define whether all values are accepted, exactly one is required, or repetition is an error; do not silently take the first value.

If the application must not expose arbitrary command-line options as Spring configuration, disable the command-line property source:

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public static void main(String[] args) {
    SpringApplication application = new SpringApplication(Application.class);
    application.setAddCommandLineProperties(false);
    application.run(args);
}

With that setting, configuration properties no longer receive command-line values automatically. Parse ApplicationArguments yourself and map only approved values into the application model. See the SpringApplication API.

Parse positional arguments and subcommands explicitly

Use ApplicationArguments when the interface includes a command word or positional values. Validate the complete shape before dispatching to business operations:

@Bean
ApplicationRunner explicitCli() {
    return args -> {
        rejectUnknown(args);

        List<String> positional = args.getNonOptionArgs();
        if (positional.isEmpty()) {
            throw new IllegalArgumentException(
                    "Usage: app <command> [arguments]");
        }

        String command = positional.getFirst();
        switch (command) {
            case "import" -> validateImport(positional);
            case "export" -> validateExport(positional);
            default -> throw new IllegalArgumentException(
                    "Unknown command: " + command);
        }
    };
}

Convert values narrowly and report the expected format and range. Never turn malformed input into zero or a default:

static int parsePositiveInt(String value, String option) {
    try {
        int parsed = Integer.parseInt(value);
        if (parsed < 1 || parsed > 1000) {
            throw new IllegalArgumentException(
                    option + " must be between 1 and 1000");
        }
        return parsed;
    }
    catch (NumberFormatException ex) {
        throw new IllegalArgumentException(
                option + " must be a decimal integer", ex);
    }
}

For a one-shot command, document the exact invocation used in automation. Passing arguments through mvn spring-boot:run has Maven-plugin-specific handling; see the Spring Boot Maven run goal.

Finish validation before business work begins

ApplicationRunner and CommandLineRunner execute after the application context starts but before SpringApplication.run(...) completes and before Boot considers the application ready. A validation failure in a runner therefore prevents normal readiness. It does not undo infrastructure that may already have initialized during context startup, so validate as early as the architecture permits. Spring Boot recommends runners for startup tasks rather than using @PostConstruct; see the SpringApplication reference.

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Put cross-field and domain checks in a class-level constraint or an explicit preflight step. Complete all checks before opening files, connecting to databases, calling network clients, or starting subprocesses. Examples include:

• Require --cli.input for an import operation.
• Require a start date to precede an end date.
• Reject incompatible options, such as JSON output combined with a binary mode.
• Require exactly one of a file input and standard input.
• Resolve a path against an approved root and check the resolved target against filesystem policy.
• Cap requested concurrency at an operational limit.

Bean Validation checks declared constraints; it does not establish that a path is authorized, a URL is reachable, or a requested operation is safe. A normalized path may still escape through a symlink, so enforce filesystem policy at the point of use.

Keep subprocess execution safe

Arguments from scripts, schedulers, containers, or administrators are still untrusted. Avoid building a shell command by concatenating input. Prefer not to invoke an operating-system command at all; when it is necessary, use a fixed executable, a parameterized argument list, an allowlist, and least privilege:

List<String> command = List.of(
        "/usr/bin/tool",
        "--safe-mode",
        "--",
        validatedPath.toString());

Process process = new ProcessBuilder(command)
        .redirectErrorStream(true)
        .start();

Use -- only if the invoked program supports it; it commonly marks the end of option parsing so a user-controlled path cannot be interpreted as a new flag. Shell metacharacter filtering alone is not sufficient. OWASP distinguishes shell command injection from argument injection: an input may become a dangerous option even if it cannot start another shell command. Follow the OWASP OS Command Injection Defense Cheat Sheet and MITRE CWE-78 guidance on allowlisting and isolation.

Do not place passwords, tokens, or private keys in arguments when avoidable. Process inspection, supervisors, crash reports, and diagnostics may expose them; use an appropriate secret store or protected mounted file instead.

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Make failures useful to people and automation

Return a non-zero status for invalid input and make the message actionable without echoing secrets or dumping the raw argument array. Keep error categories distinct where callers need to respond differently:

• Usage or validation error.
• Authentication or authorization failure.
• Input file or network failure.
• Internal failure.

For example: Invalid --cli.count: expected an integer from 1 through 1000. Spring Boot provides startup failure analyzers and supports ExitCodeGenerator; the exit-code number is the application’s contract, not a universal Spring Boot value. See the startup failure and exit documentation and ExitCodeGenerator API.

A small wrapper can choose a documented usage-error code, for example 2, but that convention is yours to define:

public static void main(String[] args) {
    try {
        SpringApplication.run(Application.class, args);
    }
    catch (RuntimeException ex) {
        System.err.println(ex.getMessage());
        System.exit(2);
    }
}

For a managed shutdown path, Spring Boot also supports SpringApplication.exit(...) with an ExitCodeGenerator; pass the resulting code to System.exit as documented in the SpringApplication reference.

Test the input contract and failure boundary

Test that invalid input fails before any operation with side effects, not just that a constraint throws. A useful test set includes:

• A valid invocation and each documented default.
• Missing required values, a bare flag, and an empty assignment such as --cli.operation=.
• Malformed numbers, values at and beyond each bound, and overflow.
• Repeated options and unknown names both inside and outside the cli prefix.
• Positional arguments with missing, extra, or reordered values; unknown subcommands; and the documented behavior of --.
• Cross-field conflicts and paths that traverse outside the approved root, including symlink cases where relevant.
• Hostile subprocess values that resemble shell syntax or options, verifying they remain one data argument and cannot change the invoked tool’s behavior.
• Exit status and error output, including confirmation that secrets and the raw argument list are not printed.

Also check resource limits on string lengths, list sizes, file sizes, and numeric inputs; define explicit locale and format rules for dates or decimals; and restrict or normalize identifiers where Unicode lookalikes could change authorization or routing. Lazy initialization may defer unrelated configuration failures, so do not rely on it as an argument-safety mechanism. If several runners exist, order validation ahead of runners that perform work using @Order or Ordered.

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